test-js: Add a mark_as_garbage method to force GC to collect that object

This should fix the flaky tests of test-js.
It also fixes the tests when running with the -g flag since the values
will not be garbage collected too soon.

Userland/Libraries/LibJS/Heap/Heap.h

@@ -83,6 +83,8 @@ public:
 
     BlockAllocator& block_allocator() { return m_block_allocator; }
 
+    void uproot_cell(Cell* cell);
+
 private:
     Cell* allocate_cell(size_t);
 
@@ -117,6 +119,8 @@ private:
 
     WeakContainer::List m_weak_containers;
 
+    Vector<Cell*> m_uprooted_cells;
+
     BlockAllocator m_block_allocator;
 
     size_t m_gc_deferrals { 0 };