fixes #35519

This PR is a refactor of Spack's stand-alone test process to be more spack- and pytest-like. It is more spack-like in that test parts are no longer "hidden" in a package's run_test() method and pytest-like in that any package method whose name starts test_ (i.e., a "test" method) is a test part. We also support the ability to embed test parts in a test method when that makes sense.

Test methods are now implicit test parts. The docstring is the purpose for the test part. The name of the method is the name of the test part. The working directory is the active spec's test stage directory. You can embed test parts using the test_part context manager.

This PR adds other functionality (in part because it was originally tested here).

PR highlights:

• adds support for multiple test_* stand-alone package test methods, each of which is an implicit test_part for execution and reporting purposes;
• deprecates package use of run_test();
• exposes some functionality from run_test() as optional helper methods;
• adds a SkipTest exception that can be used to flag stand-alone tests as being skipped;
• updates the packaging guide section on stand-alone tests to provide more examples (per feedback from multiple people);
• restores the ability to run tests "inherited" from provided virtual packages (was also in Bugfix: restore ability to run virtual tests #35380);
• prints the test log path (like we currently do for build log paths) (was also in Output test log path (optionally contents) post-install, include post-install times #35315);
• times and reports the post-install process (since it can include post-install tests) (was also in Output test log path (optionally contents) post-install, include post-install times #35315); and
• corrects context-related error message to distinguish test recipes from build recipes.