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

tests/run-tests.py

@@ -2246,15 +2246,19 b' class TestResult(unittest._TextTestResul'
         # os.times module computes the user time and system time spent by
         # child's processes along with real elapsed time taken by a process.
         # This module has one limitation. It can only work for Linux user
-        # and not for Windows.
+        # and not for Windows. Hence why we fall back to another function
+        # for wall time calculations.
         test.started_times = os.times()
+        # TODO use a monotonic clock once support for Python 2.7 is dropped.
+        test.started_time = time.time()
         if self._firststarttime is None:  # thread racy but irrelevant
-            self._firststarttime = test.started_times[4]
+            self._firststarttime = test.started_time
 
     def stopTest(self, test, interrupted=False):
         super(TestResult, self).stopTest(test)
 
         test.stopped_times = os.times()
+        stopped_time = time.time()
 
         starttime = test.started_times
         endtime = test.stopped_times
@@ -2264,9 +2268,9 b' class TestResult(unittest._TextTestResul'
                 test.name,
                 endtime[2] - starttime[2],  # user space CPU time
                 endtime[3] - starttime[3],  # sys  space CPU time
-                endtime[4] - starttime[4],  # real time
-                starttime[4] - origin,  # start date in run context
-                endtime[4] - origin,  # end date in run context
+                stopped_time - test.started_time,  # real time
+                test.started_time - origin,  # start date in run context
+                stopped_time - origin,  # end date in run context
             )
         )