[wasm] Reduce run-time of GC stress test.

R=clemensh@chromium.org

Bug: 
Change-Id: I0fb6bdba7462acd1a6f86a9a378238dcb466a9e1
Reviewed-on: https://chromium-review.googlesource.com/571012Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46682}

test/mjsunit/wasm/gc-stress.js

@@ -30,8 +30,8 @@ function run(f) {
 }
 
 (function test() {
-  for (var i = 0; i < 100; i++) {
-    run(x => (x + 19));
+  for (var i = 0; i < 10; i++) {
+    run(x => (x + 19 + i));
     run(x => (x - 18));
   }
 })();