Utilize WordPress PHPUnit Test Library Included By wp-env

[ObliviousHarmony]

What?

This makes the appropriate changes to move us away from needing wp-phpunit/wp-phpunit. In the process, I also found that Yoast/PHPUnit-Polyfills was not being fully utilized. One of the changes was to change methods like setUp() to set_up() to prevent PHP 8 compatibility issues.

In addition, as per #39863, the phpunit container does not work on M1 chips. This pull request replaces the usage of the phpunit container with direct usage of vendor/bin/phpunit from Gutenberg.

Closes #39863.

Why?

#41780 made it so that there's a version of WordPress' PHPUnit test files available within the wp-env container. We should dogfood this implementation and use the test files here.

How?

This pull request makes a few changes to methods in order to be compatible with Yoast/PHPUnit-Polyfills:

* `setUp()` -> `set_up()`
* `tearDown()` -> `tear_down()`
* `setUpBeforeClass()` -> `set_up_before_class()`
* `setUpAfterClass()` -> `set_up_after_class()`

This also removes the wp-phpunit/wp-phpunit dependency since it's no longer necessary.

We also add the wptests_ database table prefix as we are running the test suite in the tests-wordpress container rather than the phpunit container.

Testing Instructions

1. Make sure CI passes because it's using the same test script.
2. Run npm run test-unit-php and verify that it passes.
3. Run npm run test-unit-php-multisite and verify that it passes.
4. Play around with different WordPress versions (it looks like we're not compatible as far back as we claim!)

[ObliviousHarmony]

I chose 6.5 here because of the stated requirements for the minimum PHP version. I wanted a version that was compatible with more versions of PHP.