Build/Test Tools: Switch back to running the PHPUnit test suite against the src directory instead of build.
Some PHPUnit tests were concerned with the state of files in the build directory. In order to allow the tests to run without requiring a build to be run first, these have been moved into assertions that run after the build step (and therefore cause it to fail if they do not pass), or into QUnit tests as necessary.
Various other PHPUnit tests implictly depend on built JavaScript files being present. These files are now touched during the test setup to avoid PHP warnings if the build files are not present.
The wp-tests-config-sample.php file and the GitHub Actions configuration have also been changed so ABSPATH uses src instead of build, therefore allowing the PHPUnit tests to be run without a build having to be run first. This means all new local installations of WordPress will use src for PHPUnit testing. If you would like to switch your existing installation over then change the location of ABSPATH in wp-tests-config.php to point to src instead of build.
Props peterwilsoncc, iandunn, gziolo, desroj, johnbillion
Fixes #51734
See #45863