Use stub_const instead of stubbing any instance

In the GarbageCollectionProfiler spec refactor the code to stub the
constant directly rather than mock a private method on the
GarbageCollectionProfiler class.

This makes it more resilient to change and mocks the to be mocked object
directly.

spec/lib/appsignal/garbage_collection_profiler_spec.rb

@@ -3,9 +3,7 @@
   let(:profiler) { described_class.new }
 
   before do
-    allow_any_instance_of(described_class)
-      .to receive(:internal_profiler)
-      .and_return(internal_profiler)
+    stub_const("GC::Profiler", internal_profiler)
   end
 
   context "on initialization" do