Add Scroll Event Test Coverage

[jgehrig]

Adds test coverage for QWheelEvent to Neovim input string conversion.

There have been a handful of scrolling related issues lately: #784 #785 #696 #630

Following the pattern we used to fix keyboard input, we should add test coverage for mouse/scrolling input.

These event conversions should be deterministic and highly testable. If we can capture several input samples from users, we can ensure this area works properly and does not suffer future regressions.