[heap][test] Set FLAG_local_heaps first before creating context

FLAG_local_heaps needs to be set before creating the context, otherwise
the StressConcurrentAllocatorTask is already started. Setting the flag
then races with background thread, which checks FLAG_local_heaps while
creating LocalHeap.

Bug: v8:10315
Change-Id: If6de748ec174dffb94a2582d24e37d24586ee95a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2404823Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69850}

test/cctest/test-concurrent-descriptor-array.cc

@@ -70,8 +70,8 @@ class ConcurrentSearchThread final : public v8::base::Thread {
 
 // Uses linear search on a flat object, with up to 8 elements.
 TEST(LinearSearchFlatObject) {
-  CcTest::InitializeVM();
   FLAG_local_heaps = true;
+  CcTest::InitializeVM();
   Isolate* isolate = CcTest::i_isolate();
 
   std::unique_ptr<PersistentHandles> ph = isolate->NewPersistentHandles();