[heap] Use ManualGCScope for test with weak handle

Otherwise concurrent allocation might start incremental marking, which
would then mark the global handle.

Bug: v8:10315
Change-Id: Ibc681b001847a7c52e9fd8a0420e42a0d0ecfbda
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2424004Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70054}

test/cctest/heap/test-heap.cc

@@ -546,7 +546,7 @@ TEST(WeakGlobalUnmodifiedApiHandlesScavenge) {
 }
 
 TEST(WeakGlobalHandlesMark) {
-  FLAG_stress_incremental_marking = false;
+  ManualGCScope manual_gc_scope;
   CcTest::InitializeVM();
   Isolate* isolate = CcTest::i_isolate();
   Factory* factory = isolate->factory();