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

tests/SConscript

@@ -205,7 +205,7 @@ if env['FULL_SYSTEM']:
 			'twosys-tsunami-simple-atomic']
 
 else:
-    configs += ['simple-atomic', 'simple-timing', 'o3-timing']
+    configs += ['simple-atomic', 'simple-timing', 'o3-timing', 'memtest']
 
 cwd = os.getcwd()
 os.chdir(str(Dir('.').srcdir))