By ensuring that tests take longer than a couple of seconds we eliminate
systematic errors in our measurements. However, we also effectively
eliminate the synchronisation overhead. To compensate, we attempt to
estimate the overhead by reporting the difference between a single
instance and the minimum averaged instance.
This test is really just for hammering the double to fixed-point conversion
(in _cairo_fixed_from_double) that happens as doubles from API calls gets
translated into internal cairo fixed-point numbers.
