[rubygems/rubygems] Silence deprecations when gemdeps is used in tests

Because we can't control 3rd party gems using deprecated rubygems
behavior, and thus outputting warnings to the screen.

https://github.com/rubygems/rubygems/commit/6912ebf20a

test/rubygems/test_gem.rb

@@ -1788,9 +1788,11 @@ You may need to `gem install -g` to install missing gems
 
     EXPECTED
 
+    Gem::Deprecate.skip_during do
       assert_output nil, expected do
         Gem.use_gemdeps
       end
+    end
   ensure
     ENV['RUBYGEMS_GEMDEPS'] = rubygems_gemdeps
   end