horridly inefficient hack in regrtest's Compare class, but it's about as
clean as can be:  regrtest has to set up the Compare instance before
importing a test module, and by the time the module *is* imported it's too
late to change that decision.  The good news is that the more tests we
convert to unittest and doctest, the less the inefficiency here matters.
Even now there are few tests with large expected-output files (the new
cost here is a Python-level call per .write() when there's an expected-
output file).