[add] unit test for phpDocumentor\Plugin\Core\ServiceProvider::register()

This is the test I wrote during the July test fest with PFZ, so be gentle :)
this appears to be dead code, I can't see it being used anywhere else in this class. 
Also: a fixture is your subject of testing (in this case that would be the ServiceProvider class); generally that is registered in the setUp method because 

a. that is re-used in every test (you are always testing your subject)
b. any dependencies (none in this case) can be mocked once in the setup and (when assigned to a property) be put in the right state in the individual tests