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

Built from https://develop.svn.wordpress.org/trunk@50441


git-svn-id: http://core.svn.wordpress.org/trunk@50052 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.8-alpha-50439';
+$wp_version = '5.8-alpha-50441';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.