Simple API tests

[timobrembeck]

Motivation

At the moment, we postponed all "real" testing to the Version 1.2 milestone (see #765).
However, before we release the CMS in production, we should have at least a few basic API tests since this is the most critical part of the system and the it's not possible to use the apps when our API isn't working properly.

Proposed Solution

For our test data, define the expected JSON result for each API endpoint and add unit tests to check whether they're equivalent.
If the test data set is missing some models (which means that the API endpoint would return an empty result), add some dummy data to the test data before defining the expected result.

Alternatives

Additional Context

While you're at it, you could also review our basic view-tests in integreat_cms/cms/tests/views and check whether there are any views which can be added without much effort (e.g. because they don't expect custom GET/POST params).