Tests for Source Code Loaders

[swissspidy]

Extracted from #70:

We don't want to test if Git/Mercurial/Subversion are actually working, but whether the behavior of the classes is correct. Otherwise tests fail if the software isn't installed on your machine (or the CI provider).

I've looked into https://github.com/php-mock/php-mock-phpunit and https://github.com/mikehaertl/php-shellcommand for mocking exec() calls and some sort of Command dependency injection.