[test] Disable test with FLAG_stress_concurrent_allocation

The test reads slack properties after the end of the object. With
FLAG_stress_concurrent_allocation a background thread allocates a lot
of objects and might cause concurrent sweeping which causes a TSAN
race. Since this only happens with this test, disable the test in this
configuration.

Bug: v8:11698
Change-Id: If922a4a233006d3719294ae2f3a05820117e9250
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2856536Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74419}

test/cctest/test-inobject-slack-tracking.cc

@@ -839,6 +839,8 @@ TEST(ObjectLiteralPropertyBackingStoreSize) {
 }
 
 TEST(SlowModeSubclass) {
+  if (FLAG_stress_concurrent_allocation) return;
+
   // Avoid eventual completion of in-object slack tracking.
   FLAG_always_opt = false;
   CcTest::InitializeVM();