Add short memtest run to quick regressions.

Add short memtest run to quick regressions.
Caveats:
- Even though memtest is ISA-independent, it will only
run for the Alpha builds, since there's no way to specify
ISA-independent reference files and I didn't want to commit
3 copies since I'm not sure we want to run it for all the
different ISAs anyway.
- Reference outputs were generated on my laptop,
so performance numbers will be low compared to zizzer.

--HG--
extra : convert_revision : 210fe4abecc19fbab0b15402c6cb4863650bab66