Make regrtest complain when -M and -j are used together.

Make regrtest complain when -M and -j are used together.

-j doesn't pass the memlimit on to child processes, so this doesn't work at
present, and even if it did, running multiple bigmem tests at once would
usually not be desirable (since you generally want to devote as much of the
available RAM as possible to each test).