tests: driver: nrf_clock_control: Verify that clocks are ready

Ensure that a given clock controller is ready before making requests
to it. Otherwise, if for some reason the clock controller fails to
initialize (for example, when BICR turns out to be not populated with
required values), the test may end up with an enigmatic bus fault.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>