tests: kernel: workq: work: Fix potential race in the test

Test was setting up timer for 1 system tick and then work was
cancelled. It was assumed that work will be cancelled before
timer expires. This is the case for low frequency system clock
(e.g. qemu targets using 100Hz) but there are cases when system
clock has higher frequency (32kHz on nRF). In that case, timer
was occasionally expiring before cancellation and test was
randomly failing.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>