`spack test` has a spurious '[+] ' in the output:

```
lib/spack/spack/test/install.py .........[+] ......
```

Output is properly suppressed:

```
lib/spack/spack/test/install.py ...............
```

Makes the following changes:

* (Fixes #15620) tty configuration was failing when stdout was 
  redirected. The implementation now creates a pseudo terminal for
  stdin and checks stdout properly, so redirections of stdin/out/err
  should be handled now.
* Handles terminal configuration when the Spack process moves between
  the foreground and background (possibly multiple times) during a
  build.
* Spack adjusts terminal settings to allow users to to enable/disable
  build process output to the terminal using a "v" toggle, abnormal
  exit cases (like CTRL-C) could leave the terminal in an unusable
  state. This is addressed here with a special-case handler which
  restores terminal settings.

Significantly extend testing of process output logger:

* New PseudoShell object for setting up a master and child process
  and configuring file descriptor inheritance between the two
* Tests for "v" verbosity toggle making use of the added PseudoShell
  object
* Added `uniq` function which takes a list of elements and replaces
  any consecutive sequence of duplicate elements with a single
  instance (e.g. "112211" -> "121")

Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>

The performance improvements done in #14693 where leaving the DB in an inconsistent state when specs were removed from it. This PR updates the DB internal state whenever the DB is written to a file.

Note that we still cannot properly enumerate installed dependents, so there is a TODO in this code. Fixing that will require the dependents dictionaries in specs to be re-keyed (either by hash, or not keyed at all -- a list would do).  See #11983 for details.

Reading the database repeatedly can be quite slow.  We need a way to speed
up Spack when it reads the DB multiple times, but the DB has not been
modified between reads (which is nearly all the time).

- [x] Add a file containing a unique uuid that is regenerated at database
    write time. Use this uuid to suppress re-parsing the database
    contents if we know a previous uuid and the uuid has not changed.

- [x] Fix mutable_database fixture so that it resets the last seen
    verifier when it resets.

- [x] Enable not rereading the database immediately after a write. Make
    the tests reset the last seen verifier in between tests that use the
    database fixture.

- [x] make presence of uuid module optional

Removed the code that was converting the old index.yaml format into
index.json. Since the change happened in #2189 it should be
considered safe to drop this (untested) code.

If a user invoked "spack env activate example-henv", Spack would
mistakenly interpret the "-h" from "example-henv" as the "-h" option.
This commit allows users to create and activate environments with
"-h" in the name.

This issue existed for bash shell support as well as csh support, and
this commit addresses both, along with some other unrelated csh
support issues.

* only override spec prefix for non-external packages

* add test that environment shell modifications respect explicitly-specified prefixes for external packages

* add clarifying comment

spack.util.environment_after_sourcing_files compares the local
environment against a shell environment after having sourced a
file; but this ends up including the default shell profile and
rc, which might differ from the local environment.

To change this, compare against the default shell environment,
expressed here as 'source /dev/null'.

fixes #15775

Add all the variables listed here:

https://lmod.readthedocs.io/en/latest/090_configuring_lmod.html

to the list of those blacklisted when constructing environment
modifications by sourcing files.

Spack currently cannot run as a background process uninterrupted because some of the logging functions used in the install method (especially to create the dynamic verbosity toggle with the v key) cause the OS to issue a SIGTTOU to Spack when it's backgrounded.

This PR puts the necessary gatekeeping in place so that Spack doesn't do anything that will cause a signal to stop the process when operating as a background process.

Bug: Spack hangs on some Cray machines

Reason: The TERM environment variable is necessary to run bash -lc "echo $CRAY_CPU_TARGET", but we run that command within env -i, which wipes the environment.

Fix: Manually forward the TERM environment variable to env -i /bin/bash -lc "echo $CRAY_CPU_TARGET"

When trying to use an upstream Spack repository, as of f2aca865 Spack
was attempting to write to the upstream DB based on a new metadata
directory added in that commit. Upstream DBs are read-only, so this
should not occur.

This adds a check to prevent Spack from writing to the upstream DB

fixes #15449

Before this PR a call to pkg.url_for_version was modifying
class attributes determining different results for subsequents
calls and an error when the urls was empty.

* bugfix: ensure bootstrapped compilers built before packages using the compiler

This is a minor permission fix on the new installer.py introduced in #13100.

This recovers the old behavior of replace_prefix_bin that was
modified to work with elf binaries by prefixing os.sep to new prefix
until length is the same as old prefix.

fixes #15184

GraphBLAS depends on m4 according to CMake error message
Do not use INSTALL= when compiling the library

Testing the install StopIteration exception resulted in an attribute error:

AttributeError: 'StopIteration' object has no attribute 'message'

This PR adds a unit test and resolves that error.

* Recover coverage from subprocesses during unit tests

The new build process, introduced in #13100 , relies on a spec's dependents in addition to their dependencies. Loading a spec from a yaml file was not initializing the dependents.

- [x] populate dependents when loading from yaml

The distributed build PR (#13100) -- did not check the install status of dependencies when using the `--only package` option so would refuse to install a package with the claim that it had uninstalled dependencies whether that was the case or not.

- [x] add install status checks for the `--only package` case.
- [x] add initial set of tests

* Fix for being able to 'spack load' packages that have been renamed.

* tests: add test for 'spack load' of a installed, but renamed/deleted package

This change stores packages' configure arguments during build and makes
use of them while refreshing module files. This fixes problems such as in
#10716.

Newer versions of LLVM provide a one-line command to set up LLDB code
signing. Now the build will abort only if this command fails.

https://lldb.llvm.org/resources/build.html#code-signing-on-macos