Unit tests: Support unit tests in extensions and skins

Unit tests: Support unit tests in extensions and skins

For now only extensions with skin/extension.json which follow PSR-4 are
supported.

DefaultSettings is loaded in bootstrap to work around errors when unit tests are
run followed by integration tests (e.g. `vendor/bin/phpunit`)

Usage:

 - composer phpunit:unit will run all unit tests in core and any tests in
   `{extensions/skins}/tests/phpunit/unit` which also extend
   MediaWikiUnitTestCase, and which have an extension.json file
- Pass a specific directory to only run unit tests for a particular extension,
   e.g. `composer phpunit:unit -- extensions/GrowthExperiments`

Bug: T226911
Change-Id: I237a9f82e4d1b05cf2f08b3e4bb7ffcd8d47111c