Use capture_output instead of assert_silent

Author: hsbt

test/rubygems/test_gem_util.rb

@@ -15,10 +15,12 @@ def test_class_popen
   def test_silent_system
     skip if Gem.java_platform?
     Gem::Deprecate.skip_during do
-      assert_silent do
+      out, err = capture_output do
         Gem::Util.silent_system(*ruby_with_rubygems_in_load_path, '-e', 'puts "hello"; warn "hello"')
       end
     end
+    assert_empty out
+    assert_empty err
   end
 
   def test_traverse_parents