[test] Only update FLAG_local_heaps if still disabled

Avoid race with concurrent allocation thread by updating the flag
conditionally.

Bug: v8:10315
Change-Id: I63df8ab5a6a506e9337281c9fef7555b9deaaee6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2424264Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70058}

test/cctest/compiler/serializer-tester.cc

@@ -25,7 +25,7 @@ SerializerTester::SerializerTester(const char* source)
   // --local-heaps is enabled by default, but some bots disable it.
   // Ensure that it is enabled here because we have reverse implication
   // from --no-local-heaps to --no-concurrent-inlining.
-  FLAG_local_heaps = true;
+  if (!FLAG_local_heaps) FLAG_local_heaps = true;
   // The tests don't make sense when optimizations are turned off.
   FLAG_opt = true;
   // We need the IC to feed it to the serializer.