[rubygems/rubygems] Properly load the generated gemspec

The previous code loads bundler's gemspec which does not include the generated
gemspec file, and thus the test was passing where it should indeed fail.
With this change, the test properly fails now.

https://github.com/rubygems/rubygems/commit/2098ea0d75

spec/bundler/commands/newgem_spec.rb

@@ -633,7 +633,7 @@ RSpec.describe "bundle gem" do
     it "does not include the gemspec file in files" do
       bundle "gem #{gem_name}"
 
-      bundler_gemspec = Bundler::GemHelper.new(gemspec_dir).gemspec
+      bundler_gemspec = Bundler::GemHelper.new(bundled_app(gem_name), gem_name).gemspec
 
       expect(bundler_gemspec.files).not_to include("#{gem_name}.gemspec")
     end