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.
Merge pull request #1734 from Ichimonji10/test-repository

Deduplicate test data
Tweak smoke test "get_api_links"

Make the errors produced by the test more useful. For example, this comparison:

    assertEqual(frozenset(foo.keys()), frozenset(bar.keys()))

produces much nicer error messages than this comparison:

    for k in foo.keys():
        self.assertIn(k, bar)

Rename several variables for the sake of clarity. For example, use this
assertion:

    self.assertEqual(
        frozenset(api_paths[group]),
        frozenset(API_PATHS[group])
    )

Instead of this assertion:

    self.assertEqual(frozenset(paths), frozenset(API_PATHS[category]))