tests: kernel: workq: work: Tweak test for fast system clock

Test assumes that system clock is slow enough so 1 tick timeout
will not expire before k_timer_start function exists. That is
not the case when system clock is fast (relatively to the cpu
clock). Increase the timeout and add synchronization point.

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