Add unit/regression tests

[minisbett]

Adds unit tests covering all of osu-native.

The unit test interacts with osu-native by calling the high-level [OsuNativeFunction] methods, rather than using the FFI. This is deemed okay, as the concept of FFI should always reliably work, and the error-handling that differentiates the higher-level functions from the native ones is deterministic and tested as well.

Coverage:

• RulesetTests
• BeatmapTests
• ModTests
• ModsCollectionTests
• OsuDifficultyCalculatorTests
• TaikoDifficultyCalculatorTests
• CatchDifficultyCalculatorTests
• ManiaDifficultyCalculatorTests
• OsuPerformanceCalculatorTests
• TaikoPerformanceCalculatorTests
• CatchPerformanceCalculatorTests
• ManiaPerformanceCalculatorTests
• ErrorHandlerTests
• ManagedObjectStoreTests (including handles)

TODO:

• Add xmldocs with more elaborate expectations to every test