refactor test_catalog

- Tests for "missing" attributes are actually testing with null values,
  not missing keys.
- Not being able to explicitly provide alternative URLs resulting in
  hacky workarounds that can be avoiding by simply exposing kwargs with
  reasonable defaults
- Several tests redundantly asserted the expected response status, which
  is already handled by the test framework

Change-Id: Icadb4927bcf03ca7e79fef1507e419a4932f32a9
Related-Bug: 1250765
Merge "refactor test_catalog"