add test/extension to Makefile, equivalent to phpunit --group extension
remove test already covered on ReleaseListTest
fix ReleaseListTest so it won't fail everytime php-releases.json is renewed
annotate all tests with `@small`, `@large`, `@group extension`, `@group command`

- test quick things: phpunit --group small
- test slow operations: phpunit --group large
- test just commands (slow): phpunit --group command
- test just extensions: phpunit --group extension