[heap] Fix CompactionSpace test and move to unittests

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 40bff399473633efc8a9a88dd7499f359779c9af..4151fb97af713362cb3f5d5c65fdb83fda5d9c2e 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -746,8 +746,8 @@ class Heap {
   // when introducing gaps within pages. If slots could have been recorded in
   // the freed area, then pass ClearRecordedSlots::kYes as the mode. Otherwise,
   // pass ClearRecordedSlots::kNo.
-  HeapObject* CreateFillerObjectAt(Address addr, int size,
-                                   ClearRecordedSlots mode);
+  V8_EXPORT_PRIVATE HeapObject* CreateFillerObjectAt(Address addr, int size,
+                                                     ClearRecordedSlots mode);
 
   bool CanMoveObjectStart(HeapObject* object);