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]))