tests: drivers: Turn on HFCLK before LFCLK calibration in nRF devices

This change plays with situation in nRF54L devices where XOTUNE make
LF clock calibration process starts a bit longer. Now HF clock
is configured to be ready right before LF calibration test.

Signed-off-by: Karol Lasończyk <karol.lasonczyk@nordicsemi.no>