Oops we were accumulating paths during each spiral iteration and so the
tests were getting slower and slower and slower...
[And fix a couple of other instances of path accumulation.]
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.
We don't have one just for this purpose. The only other
path with many intersections that gets actually rendered is zrusin-another,
but that might be sped up in the future (say by identifying
collinearities up front or something like that.)
