Add unit tests for game endpoints

[pavog]

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

• getGames(): default pagination (index=0, pageSize=50), a custom offset and page size, and mapping the results to Game
• getGame()
• getGameVersionTypes()
• getGameVersions(): the request goes to /v2/games/{gameId}/versions
• getGameVersionsByType(): filters the v2 response by type

The fixtures in tests/Unit/Client/Fixtures are recorded responses from the real CurseForge API.