More tests for ASST3.2

Added 2 new sbrktests, one to test a huge sbrk with only touch a few pages (shouldn't crash),
and another to make sure pages are getting freed when the breakpoint moves down. Also, added
a stacktest that checks that 4M stacks are supported and pages are allocated on-demand.

test161/commands/vm.tc

@@ -8,6 +8,7 @@ templates:
   - name: /testbin/parallelvm
   - name: /testbin/sbrktest
   - name: /testbin/sort
+  - name: /testbin/stacktest
   - name: /testbin/zero
 
 #Triples