[TASK] Standalone functional test runner

Introduce base code for a standalone php test runner.

"./typo3conf/ext/phpunit/Composer/vendor/bin/phpunit
	-c typo3/sysext/core/Build/FunctionalTests.xml"

This way phpunit is called fully standalone and does not
depend on TYPO3 CMS bootstrap itself.

Resolves: #51111
Releases: 6.2
Change-Id: Ie790eec315f7a273fc62856df3169aa56b2ad60d