tests/kernel/workq/work_queue: Fix for fast/non-standard tick rates

This test was written to properly align its millisecond-measured wait
time and assumed that there would be no other overhead.  In fact on
fast tick rate systems (or even ones where the alignment computation
doesn't provide the needed padding as "slop") that's not quite enough
time to complete the full test.  There are cycles between the sleep
calls that need to be accounted for, and aren't.

Just give it one extra work item of time before failing.  We aren't
testing work queue timing precision here, just evaluation semantics.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>