tests: Allow some difference in rounding in timeval test.

tests: Allow some difference in rounding in timeval test.

We now use a time source that provides nanosecond granularity.
However, our test uses gettimeofday() for comparision, which has
microsecond granularity.  In some cases this can lead to different
values depending on the rounding.  This allows us to be off by one
to prevent intermittent test failures.