tests/kernel/context: Improve to resist parallel threads interrupting

This test assumes that nothing will wake the CPU apart from
the timer set by the test.
But that is not necessarily the case.
Some other platform thread started at boot
may be waking the CPU every now and then.

Let's allow for some spurious wakes while we are testing k_cpu_idle,
while at the same time ensuring we are not just busy waiting
all the way.

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