Fix back-branch pg_regress scripts to try the "canonical" expected file if we

Fix back-branch pg_regress scripts to try the "canonical" expected file if we
tried a variant file from resultmap and it didn't match.  This is already done
in HEAD's C-code version, and is needed because OpenBSD has recently migrated
to a more standard handling of float underflow --- see buildfarm results
from emu.