tests: Modifier and group latch - Added a big bunch of tests for modifier latch. No yet exhaustive, but should cover the most usual use cases. - Added missing test cases for breaking the group latch. Ideally, more tests should be added to match the coverage of modifiers latches. WARNING: it is ambiguous what prevents a latch when multiple keys are *simultenously* operated together. We currently assuming that any action that is not identical to the considered latch prevents it.