[heap] Disable --stress-concurrent-allocation for one more test

Test was asserting heap size before and after GC. With background
thread allocation those assertions might not hold.

Bug: v8:10315
Change-Id: I4f8c0f6d0b80040b3c89f85e801416abb29ed30e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2421999Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70034}

test/cctest/heap/test-heap.cc

@@ -5848,6 +5848,7 @@ TEST(LiveBytes) {
 
 TEST(Regress615489) {
   if (!FLAG_incremental_marking) return;
+  ManualGCScope manual_gc_scope;
   CcTest::InitializeVM();
   v8::HandleScope scope(CcTest::isolate());
   Heap* heap = CcTest::heap();