Add keyboard input tests

[healeycodes]

Add a test that covers input/actions/movement controls and checks the output is correct.

Perhaps we can manually call the update function: https://github.com/healeycodes/noter/blob/472737ff4de972ce7042463814ab8ded4370dfc3/editor.go#L743

And mock out ebiten.IsKeyPressed?

This would have really helped with edge cases in https://github.com/healeycodes/noter/pull/9