[heap] Use ManualGCScope for test

Ensures that there is no concurrent allocation happening.

Bug: v8:10315
Change-Id: Ief40cbde9d859e3a2eea66d6e4437d7f0e3840e8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2418951Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69998}

test/cctest/heap/test-heap.cc

@@ -6517,7 +6517,7 @@ TEST(RememberedSetRemoveRange) {
 
 HEAP_TEST(Regress670675) {
   if (!FLAG_incremental_marking) return;
-  FLAG_stress_incremental_marking = false;
+  ManualGCScope manual_gc_scope;
   CcTest::InitializeVM();
   v8::HandleScope scope(CcTest::isolate());
   Heap* heap = CcTest::heap();