Improve stats.proc_stats() unit testing.

testStats.py:
- Parameterize proc_stats() test and verification.
- Add MockTime class to manipulate time.time() to improve processing-
latency testing.

testutils.py:
- Add filename_to_time() and dirname_to_datetime().
Add images processed per minute to stats.

- Update unit test.
- Simplify proc_stats() code slightly.