[vm,gardening] Reduce amount of work performed in vm/dart/gc/chunked_binary_trees_test

The test vm/dart/gc/chunked_binary_trees_isolates_test which uses
vm/dart/gc/chunked_binary_trees_test has been timing out after 4m on
vm-aot-android-release-arm_x64 despite being declared as Slow already.

TEST=ci

Change-Id: I19202b681c81157cfd01f2090f36a7159b12718f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/437889
Auto-Submit: Alexander Markov <[email protected]>
Commit-Queue: Ryan Macnak <[email protected]>
Reviewed-by: Ryan Macnak <[email protected]>

Author: alexmarkov

runtime/tests/vm/dart/gc/chunked_binary_trees_test.dart

@@ -112,7 +112,7 @@ runBottomUp(int depth) {
 }
 
 main() {
-  for (var i = 0; i < 5; i++) {
+  for (var i = 0; i < 2; i++) {
     runTopDown(10);
     runBottomUp(10);
   }