clear() should be overriden by all test bench elements, that do not support it natively

[stefanuebe]

Some testbench elements have already their own implementation of clear(), like the combobox, but some do not and thus run into an InvalidElementStateException, like the timepicker or checkbox group.

These elements should override that method to prevent (especially new) devs from the need to investigate potential and not so obvious issues with our test framework.