tests: drivers: counter: Add delay for clock stabilization

On nRF platforms, Xtal LF clock source starts hundreds of milliseconds.
Until it is not started, test may fail due to wrong timing. Add initial
delay in the test for required clocks complete their start-up.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>
Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>