Add unit tests for category endpoints

[pavog]

Depends on #15

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This PR adds unit tests for the category endpoints of CurseForgeAPIClient. They run against the mocked HTTP client from ClientTestCase.

• getCategories(): only gameId is sent by default, without classId or classesOnly. The results are mapped to Category.
• getCategories() with classId and classesOnly: classesOnly is sent as true (see Fix boolean query parameters #14)
• getCategory(): requests all categories of the game and returns the one with the given id
• getCategory() with an unknown id returns null

The fixture getCategories.json is a recorded response from the real CurseForge API, shortened to a few entries.