Deduplicate test data

Create function `_test_data` in module `tests.foreman.api.test_repository`. Use
this function to feed data to several DDT data decorators, like this:

    @ddt.data(*_test_data())
    def test_foo(self, data):
        ...

This allows test data to be written out once within the body of that function
and then re-used in several places.