docs: update PHPUnit testing README.md with corrections #62065

tests/phpunit/README.md

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+# PHPUnit Tests
+
+PHPUnit is the official testing framework chosen by the core team to test our PHP code.
+
+This documentation will assume you have a local development environment of your own choosing. For other testing workflows, such as Docker, see the [PHP: PHP Unit](https://make.wordpress.org/core/handbook/testing/automated-testing/phpunit/) handbook page.
+
+## Setup
+
+1. Check out the wordpress-develop repository
+
+       git clone git://develop.git.wordpress.org/ wordpress-develop
+       cd wordpress-develop
+
+2. Run `composer update --with-all-dependencies`
+3. Create a clean MySQL database and user. **DO NOT USE AN EXISTING DATABASE** or you will lose data, guaranteed.
+4. Copy `wp-tests-config-sample.php` to `wp-tests-config.php`, edit it to include your testing database credentials.
+5. Run the tests from the repository root:
+   - To execute a particular test:
+
+         $ vendor/bin/phpunit tests/phpunit/tests/test_case.php
+   - To execute all tests:
+
+         $ vendor/bin/phpunit
+
+## Notes:
+
+Test cases live in the `tests` subdirectory. All files in that directory will be included by default. Extend the `WP_UnitTestCase` class to ensure your test is run.
+
+`phpunit` will initialize and install a (more or less) complete running copy of WordPress each time it is run. This makes it possible to run functional interface and module tests against a fully working database and codebase, as opposed to pure unit tests with mock objects and stubs. Pure unit tests may be used also, of course.
+
+Changes to the test database will be rolled back as tests are finished, to ensure a clean start next time the tests are run.
+
+`phpunit` is intended to run at the command line, not via a web server.