[rubygems/rubygems] Improve assertion

https://github.com/rubygems/rubygems/commit/7f2f2b898c

Co-authored-by: Martin Emde <martin.emde@gmail.com>

spec/bundler/bundler/definition_spec.rb

@@ -35,7 +35,7 @@ RSpec.describe Bundler::Definition do
 
       it "does not create a lock file" do
         subject.lock
-        expect(File.file?("Gemfile.lock")).to eq false
+        expect(bundled_app_lock).not_to be_file
       end
     end
   end