LibGC: Enforce that a Cell type must declare the allocator to use

This ensures that we are explicitly declaring the allocator to use when
allocating a cell(-inheriting) type, instead of silently falling back
to size-based allocation.

Since this is done in allocate_cell, this will only be detected for
types that are actively being allocated. However, since that means
they're _not_ being allocated, that means it's safe to not declare
an allocator to use for those. For example, the base TypedArray<T>,
which is never directly allocated and only the defined specializations
are ever allocated.

Libraries/LibTest/JavaScriptTestRunnerMain.cpp

@@ -21,6 +21,8 @@ TestRunner* ::Test::TestRunner::s_the = nullptr;
 
 namespace JS {
 
+GC_DEFINE_ALLOCATOR(TestRunnerGlobalObject);
+
 RefPtr<::JS::VM> g_vm;
 bool g_collect_on_every_allocation = false;
 ByteString g_currently_running_test;