[test] Shrink async GC stress test to avoid timeouts on the slowest bots.

TEST=ci
Bug: https://github.com/dart-lang/sdk/issues/54590
Change-Id: Ib444f831e966fc980848cdd6495a5d0a343d439d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/346581
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

runtime/tests/vm/dart/gc/chunked_binary_trees_async_test.dart

@@ -119,7 +119,7 @@ runBottomUp(int depth) async {
 
 main() async {
   for (var i = 0; i < 5; i++) {
-    await runTopDown(10);
-    await runBottomUp(10);
+    await runTopDown(9);
+    await runBottomUp(9);
   }
 }