vg_regtest: Miscellaneous tweaks

vg_regtest: Miscellaneous tweaks

Drop the requirement that there must be at least one .stderr.exp
file. That way we can eliminate some 440 empty stderr.exp files and
a few stderrB.exp files.

Drop the requirement that there must be a "filter_stderr" script.
If there is nothing to filter (e.g. iropt-test) a filter script is
not meaningful. In that is so desired, specify an empty stderr_filter
and stderrB_filter in the .vgtest file.