Switch testrunner.py to report times with monotonic clock

Bug: b/138868214
Test: art/test/testrunner/testrunner.py --target --32 --jit --target \
   --verbose --ndebug -t 159 --timeout 10
Test: art/test/testrunner/testrunner.py --target --32 --jit --target \
   --verbose --ndebug -t 159 --timeout 1

Change-Id: I3f59154a08fd84699adfc6d774c4ec3fe2729b05
1 file changed