Use fixture for test

Author: andyw8

test/setup_bundler_test.rb

@@ -484,20 +484,14 @@ def test_ensures_lockfile_remotes_are_relative_to_default_gemfile
 
   def test_ruby_lsp_rails_is_automatically_included_in_rails_apps
     Dir.mktmpdir do |dir|
+      FileUtils.mkdir("#{dir}/config")
+      FileUtils.cp("test/fixtures/rails_application.rb", "#{dir}/config/application.rb")
       Dir.chdir(dir) do
         File.write(File.join(dir, "Gemfile"), <<~GEMFILE)
           source "https://rubygems.org"
           gem "rails"
         GEMFILE
 
-        FileUtils.mkdir(File.join(dir, "config"))
-        File.write(File.join(dir, "config", "application.rb"), <<~RUBY)
-          module MyApp
-            class Application < Rails::Application
-            end
-          end
-        RUBY
-
         capture_subprocess_io do
           Bundler.with_unbundled_env do
             # Run bundle install to generate the lockfile