tests: kernel: semaphore: fix mutual exclusion test issue

Mutual exclusion test assume that the excution order of two threads like
this:
    mutual_exclusion1 -> mutual_exclusion2 -> mutual_exclusion1 ...

but some times the excution order of two threads would be this:
    mutual_exclusion1 -> mutual_exclusion2 -> mutual_exclusion2 ...

This patch increase the loop cycle, add a variable 'tmp' to store the
value of 'critical_var' before operating it.

Signed-off-by: Huifeng Zhang <Huifeng.Zhang@arm.com>