LibGC+Tests: Allow overriding Heap::the() for tests

Add a test-only helper for setting the default GC heap, and use it
from LibGC tests that create their own test heap.

Libraries/LibGC/Heap.cpp

@@ -273,6 +273,11 @@ Heap& Heap::the()
     return *s_the;
 }
 
+void Heap::set_default_heap_for_testing(Heap& heap)
+{
+    s_the = &heap;
+}
+
 Heap::Heap(AK::Function<void(HashMap<Cell*, GC::HeapRoot>&)> gather_embedder_roots)
     : m_gather_embedder_roots(move(gather_embedder_roots))
 {

Libraries/LibGC/Heap.h

@@ -46,6 +46,7 @@ public:
     ~Heap();
 
     static Heap& the();
+    static void set_default_heap_for_testing(Heap&);
 
     template<typename T, typename... Args>
     Ref<T> allocate(Args&&... args)

Tests/LibGC/TestGCContainers.cpp

@@ -33,6 +33,11 @@ static GC::Heap& test_heap()
     return *heap;
 }
 
+TEST_SETUP
+{
+    GC::Heap::set_default_heap_for_testing(test_heap());
+}
+
 class TestVisitor : public GC::Cell::Visitor {
     virtual void visit_impl(GC::Cell& cell) override { visited_cells.set(&cell); }
     virtual void visit_impl(ReadonlySpan<GC::NanBoxedValue>) override { }

Tests/LibGC/TestGCVisitor.cpp

@@ -33,6 +33,11 @@ static GC::Heap& test_heap()
     return *heap;
 }
 
+TEST_SETUP
+{
+    GC::Heap::set_default_heap_for_testing(test_heap());
+}
+
 class TestVisitor : public GC::Cell::Visitor {
     virtual void visit_impl(GC::Cell& cell) override { visited_cells.set(&cell); }
     virtual void visit_impl(ReadonlySpan<GC::NanBoxedValue> span) override { last_nan_span_size = span.size(); }