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.cpp

@@ -191,9 +191,15 @@ public:
 void Heap::mark_live_cells(const HashTable<Cell*>& roots)
 {
     dbgln_if(HEAP_DEBUG, "mark_live_cells:");
+
     MarkingVisitor visitor;
     for (auto* root : roots)
         visitor.visit(root);
+
+    for (auto& inverse_root : m_uprooted_cells)
+        inverse_root->set_marked(false);
+
+    m_uprooted_cells.clear();
 }
 
 void Heap::sweep_dead_cells(bool print_report, const Core::ElapsedTimer& measurement_timer)
@@ -327,4 +333,9 @@ void Heap::undefer_gc(Badge<DeferGC>)
     }
 }
 
+void Heap::uproot_cell(Cell* cell)
+{
+    m_uprooted_cells.append(cell);
+}
+
 }