Reduce possible timing test failures
Reduce possible timing test failures

TestFinishIsCalledWithCorrectTime was reliant on SleepMs(20) waking
up before a certain threshold was hit, which is not reliable. This
change uses the actual time of running tests as the top threshold;
not a great improvement but a good quick fix.

Fixes #90

unittest-cpp/unittest-cpp,grahamreeds/unittest-cpp,grahamreeds/unittest-cpp,unittest-cpp/unittest-cpp,grahamreeds/unittest-cpp,pjohnmeyer/unittest-cpp,jasonhorsburgh/unittest-cpp,unittest-cpp/unittest-cpp,pjohnmeyer/unittest-cpp,jasonhorsburgh/unittest-cpp