testsuite: ztress: Add test skipping if system clock is too high

When system clock is set globally for the test which is executed
on various qemu platforms it may happen that clock is set too
high compared to CPU clock frequency. In that case test may stuck.
Added check and test skipping in that case.

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