FlxKeyboard: update outdated note about function and numpad keys
Unit test improvements + some more tests for FlxSignal

- each test method now starts with "test"
- added FlxTest.delay() as an alternative to using @AsyncTest (less setup required -> cleaner code)
- more descriptive names for test functions
Small style fix
Why was it `inline`??
Merge branch 'dev' of https://github.com/HaxeFlixel/flixel into dev