tests/kernel/context: Detect spurious wakes during timeout check

In 972a4d9f
the test was relaxed to allow a possible parallel thread
waking the CPU during the idle timeout checks.

How this was done though, a system timer driver
which wakes the kernel spuriously a bit before the
correct deadline would not be detected, even
though it should.

Let's fix this, ensuring the timer has been
called when we check the wake time in inside the
expected range.

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