Simplify PHPUnit setup

[swissspidy]

I have troubles running the unit tests if the plugin-check folder isn't within wp-content/plugins of an actual WordPress installation. I don't want to change my setup because of this.

1. Remove separate build-phpunit folder.
   • We added this in the past because of different PHP version requirements.
   • Now that we require PHP 7.2+, this is no longer needed and we can simplify our testing setup.
2. Remove plugin validation in Force_Single_Plugin_Preparation
   • Runtime check unit tests call this with the basename of the Plugin Check plugin
   • In my setup, I check out the plugin-check repository somewhere else on the computer, not within a WordPress installation.
   • Because of this, the validation would fail all my tests because the plugin couldn't be found
   • I don't see a strong need for this validation
   • Outright removing it is simpler than some additional logic to make this scenario work

With these changes I can now easily run tests locally using WP_TESTS_DIR=/path/to/wordpress-develop/tests/phpunit composer test

There are still some errors and failures, particularly in Plugin_Context_Tests and Check_Context_Tests, because of plugin path differences.

[swissspidy]

Interesting test failures right now 🤔