[test] Use EnsureFlagLocalHeapsEnabled() for enabling FLAG_local_heaps

Avoid data race when enabling flag with concurrent thread that reads
that flag as well.

Bug: v8:10315
Change-Id: I9eecc48e21b1070e8db444021264eec2784f5102
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2567697
Auto-Submit: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71528}

test/cctest/test-concurrent-feedback-vector.cc

@@ -156,8 +156,8 @@ static void CheckedWait(base::Semaphore& semaphore) {
 // Verify that a LoadIC can be cycled through different states and safely
 // read on a background thread.
 TEST(CheckLoadICStates) {
+  heap::EnsureFlagLocalHeapsEnabled();
   CcTest::InitializeVM();
-  FLAG_local_heaps = true;
   FLAG_lazy_feedback_allocation = false;
   Isolate* isolate = CcTest::i_isolate();