tests: use time.time() for relative start and stop times os.times() does not work on Windows. This was resulting in the test start, stop, and duration times being reported as 0. This commit swaps in time.time() for wall clock measurements. This isn't ideal, as time.time() is not monotonic. But Python 2.7 does not have a monotonic timer that works on Windows. So it is the best we have which is trivially usable. And test times aren't terribly important, so variances due to clock skew are arguably acceptable. Differential Revision: https://phab.mercurial-scm.org/D7126