Allow overriding default run-regress-* targets

Allow overriding default run-regress-* targets

Overriding a default run-regress-* target with custom commands is already
possible and done by many tests. The fact that it currently works depends
on behavior in the BUGs section of make.1, however. The fix is not to add
commands if the target is already defined with commands.

tested by anton
correct fix from espie, ok anton