Replace callbacks with async/await in tests

[afcapel]

Avoids deep nesting, especially when some async events, like inputs in tests, are performed in sequence.

The nesting was never very readable, but since we moved away from coffeescript it became even messier with the added closing curly brackets.

Note that I've replaced most calls to defer(callback) with await nextFrame(). However, defer is implemented as a 1ms delay while nextFrame is just calling requestAnimationFrame. They should be roughly equivalent but there might be slight differences that can make the some tests flaky. I suggest patching those tests adjusting the delay if we see they are flaky.

[kevinmcconnell]

This looks great! A big improvement for readability.