Fix test_loaded_gem_types faling in test-bundled-gems (take 3)

Gem.loaded_specs does not include prism when running with `make test-bundled-gems`.
`require 'prism'; Gem.loaded_specs['prism']` returns nil.
This does not happend when running the test with `tool/test-bundled-gems.rb`.
This does not happen with RBS.
Revert again to use sig defined in RBS instead of Prism.

Author: tompng

test/repl_type_completor/test_repl_type_completor.rb

@@ -254,13 +254,12 @@ def test_doc_namespace_error
     end
 
     def test_loaded_gem_types
-      # sig directory does not exist when running with test-bundled-gems
-      omit unless Dir.exist?("#{Gem.loaded_specs['prism'].gem_dir}/sig")
+      # gem_dir/sig directory does not exist when running with `make test-bundled-gems`
+      omit unless Dir.exist?("#{Gem.loaded_specs['rbs'].gem_dir}/sig")
 
-      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