[heap] Fix test that requires manual GC scheduling

Bug: v8:13141
Change-Id: I05e905a40a572c7f85f60629b2303cd73ae06a70
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3838731
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82564}

test/unittests/heap/embedder-tracing-unittest.cc

@@ -604,6 +604,7 @@ TEST_F(EmbedderTracingTest, TracedReferenceToUnmodifiedJSObjectDiesOnFullGC) {
 TEST_F(
     EmbedderTracingTest,
     TracedReferenceToUnmodifiedJSObjectDiesOnFullGCEvenWhenPointeeIsHeldAlive) {
+  ManualGCScope manual_gcs(i_isolate());
   // The TracedReference itself will die as it's not found by the full GC. The
   // pointee will be kept alive through other means.
   v8::Global<v8::Object> strong_global;