Refactor catalog API test code
Refactor catalog API test code

* `Url` class now has a new method named `append(self, child_url)` which
  appends another `Url` object after itself. The prefix of `child_url`
  will be ignored.
* Functions testing API endpoints for children entities of campuses was
  moved to test case classes of which the type of these entities is. For
  example, test function for API endpoint which gets a department object
  under a campus object is moved from `TestCampusApi` to
  `TestDepartmentApi`.

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