Fix test_loaded_gem_types faling in test-bundled-gems

Pre-installed Prism does not have `#{prism_gem_dir}/sig` dir.
Change test_loaded_gem_types to use signature defined in RBS instead of Prism.

Author: tompng

test/repl_type_completor/test_repl_type_completor.rb

@@ -254,10 +254,9 @@ def test_doc_namespace_error
     end
 
     def test_loaded_gem_types
-      result = ReplTypeCompletor.analyze 'Prism.parse("code").', binding: binding
+      result = ReplTypeCompletor.analyze 'RBS::CLI::LibraryOptions.new.loader.', binding: binding
       candidtes = result.completion_candidates
-      assert_includes candidtes, 'success?'
-      assert_includes candidtes, 'failure?'
+      assert_includes candidtes, 'add'
     end
 
     def test_info