Expand our suite of helper test cases/mixins

[adam-thomas]

I've been finding myself copy-and-pasting quite a lot when making tests, and I think we could use more helpers along the lines of URLsMixinBase and its descendants.

We could use:

• Mixins for testing forms, including crispy_forms.
• Mixins for testing API views, especially ones built from viewsets, which have a standard setup and are the most egregiously copy-and-paste-heavy. Being able to use mixins in order to test a series of views for a particular model in just a few concise lines would be lovely.
• Mixins for testing serializers. These are also quite copy-and-paste-heavy, and long-winded with it.
• Mixins for basic model tests, ideally asserting str() and fields.

Having basic TestCases for all of these mixins (like URLsTestCaseREST) would also be stellar. I think we could save a lot of time and tedium by refactoring our tests now.

@incuna/backend thoughts/suggestions for more test mixins?

[meshy]

Sorry deleted last comment... public project and all.

[LilyFoote]

Something I think would be nice would be adding get, post, put, patch and delete methods to BaseAPIRequestTestCase and BaseRequestTestCase.

[adam-thomas]

@Ian-Foote 👍