tests/kernel/context: Fix for nrf5340bsim_nrf5340_cpunet

This test assumes nothing else is running in the background
but when the logger is on, this board will by default
start the RTC sync mechanism, which will awake the CPU
every now and then.
Let's disable this mechanism to avoid disturbing the test.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>