Merge: nitunit: set NIT_TESTING_PATH

When working with test suites, one can now use `NIT_TESTING_PATH` to retrieve the test suite path.

It can be used to access files based on the current test suite location:

~~~nit
class MyTest
	super TestSuite

    fun test_suite_path do
        assert "NIT_TESTING_PATH".environ.basename == "my_test_suite.nit"
    end
end
~~~

Useful for test suites based on model loading like in #2327.

Pull-Request: #2331