tests: timer_api: Use busy slew threshold when checking remaining ticks

On Nordic SoCs, the clock that drives the system timer and the one that
is used in busy-waiting may be significantly skewed, so the test cases
that compare durations derived from those two clocks need to take into
account a proper threshold. After the `z_timeout_expires` function was
corrected in 3d29c9fe, it turned out
that the threshold was missing in one check and the related test case
started to fail on nRF platforms. This patch adds that threshold there.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>