Ok, this time test_set* passes :)

Currently if you have large set and small set the code will do len(large) lookups in the small set.  When large is >> than small, it is cheaper to copy large and do len(small) lookups in large.  On my laptop a size difference of 4 times is a clear winner for copy+difference_update over the status quo, even for sets of millions of entries.  For more similarly sized sets (even only factor of 2 size difference) the cost of allocating a large set that is likely to be shrunk significantly is greater than the benefit.  So my patch only switches behaviour for len(x)/4 > len(y).

This patch is complementary to the patch in issue8425, I think.