Don't accidentally leave the arena in the clamped state after calling mps_arena_collect:

* In mpiscv, release the arena immediately after collecting.
* In zmess, clamp the arena immediately after creating it.

Copied from Perforce
 Change: 181094
 ServerID: perforce.ravenbrook.com

mps/code/mpsicv.c

@@ -552,6 +552,7 @@ static void *test(void *arg, size_t s)
   alignmentTest(arena);
 
   die(mps_arena_collect(arena), "collect");
+  mps_arena_release(arena);
 
   mps_free(mv, alloced_obj, 32);
   alloc_v_test(mv);

mps/code/zmess.c

@@ -58,7 +58,8 @@
  *    the last two messages (note: no "."), to test that 
  *    mps_arena_destroy copes with ungot messages.
  *
- * Each script runs in a newly created arena.
+ * Each script runs in a newly created arena. The arena is clamped so
+ * that collections only happen when the script requests them.
  *
  *
  * CODE OVERVIEW
@@ -408,6 +409,7 @@ static void testscriptA(const char *script)
   /* arena */
   die(mps_arena_create(&arena, mps_arena_class_vm(), testArenaSIZE),
       "arena_create");
+  mps_arena_clamp(arena);
 
   /* thr: used to stop/restart multiple threads */
   die(mps_thread_reg(&thr, arena), "thread");