Build/Test Tools: Add PHPUnit to Composer dev dependencies.

This helps setting up the correct version of PHPUnit to run WordPress core tests locally and for using it as source for code autocompletion in tests. Also introduces a new Composer script `test` to run the PHPUnit tests.

Props welcher, ayeshrajans, vinkla, johnbillion, Rarst, netweb, ocean90.
Fixes #46815.
Built from https://develop.svn.wordpress.org/trunk@47881


git-svn-id: https://core.svn.wordpress.org/trunk@47655 1a063a9b-81f0-0310-95a4-ce76da25c4cd