[mjsunit] Nerf shared-memory-worker-stress a little

This test fails in --stress-opt mode because backing stores of
memories/arraybuffers that are postMessage()'d leak in d8. In normal
mode, only ~16 memories are allocated, which is not enough to OOM,
but in stress mode, it can be 5x that number. Should be fixed
by upcoming ownership changes.

BUG=v8:9380
R=clemensh@chromium.org

Change-Id: Iecec07d15339cf43b23f128f13d570dfe3b32130
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1708475Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62802}

test/mjsunit/wasm/shared-memory-worker-stress.js

@@ -6,9 +6,11 @@
 
 load("test/mjsunit/worker-ping-test.js");
 
+// TODO(v8:9380): increase {numThings} and {numWorkers} when stress-opt mode
+// no longer leaks wasm memories.
 let config = {
-  numThings: 4,         // size of circular buffer
-  numWorkers: 4,        // number of workers
+  numThings: 2,         // size of circular buffer
+  numWorkers: 2,        // number of workers
   numMessages: 2000,    // number of messages sent to each worker
   allocInterval: 10001, // interval for allocating new things per worker
   traceScript: false,   // print the script