Build/Test Tools: Update in-repo PHPUnit documentation to point to a single source of truth.

Props q0rban, ironprogrammer, desrosj, peterwilsoncc, juampick.
Fixes #62065.


git-svn-id: https://develop.svn.wordpress.org/trunk@60674 602fd350-edb4-49c9-b593-d223f7449a82

Author: aaronjorbin

tests/phpunit/README.md

@@ -0,0 +1,7 @@
+# PHPUnit Tests
+
+PHPUnit is the official testing framework chosen by the core team to test PHP code.
+
+For more information, please review the relevant Core Handbook pages:
+- [PHP: PHPUnit](https://make.wordpress.org/core/handbook/testing/automated-testing/phpunit/)
+- [Writing PHP Tests](https://make.wordpress.org/core/handbook/testing/automated-testing/writing-phpunit-tests/)