[rubygems/rubygems] Make non "test_" method private

https://github.com/rubygems/rubygems/commit/912d141a35

test/rubygems/test_require.rb

@@ -532,6 +532,8 @@ class TestGemRequire < Gem::TestCase
     end
   end
 
+  private
+
   def silence_warnings
     old_verbose, $VERBOSE = $VERBOSE, false
     yield