Fixes #74

Now MockDisabler constructor has optional 2nd parameter callback which will be executed once the test ends.
We remove the listener (for old PHPUnit versions) and for newer - remove the subscriber. It stills heavily relies on the internals of PHPUnit and it might break at any point, but as so the whole package.

Tests are a bit tricky, basically we chain of three tests, so I do not need to register a mock function in set up.
First just register a mock and it should be clearer in tearDown;
so then, the second should have the counter === 1, as the mock has been disabled, and it should be removed at this point, so the 3rd test should remain with the same counter (=== 1) - and actually it was failing before before the 'callback' solution.