Add class name to assert messages

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57503 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_io.rb

@@ -3520,8 +3520,8 @@ __END__
           assert_equal before, after,
             'no strings left over after write [ruby-core:78898] [Bug #13085]'
           assert_not_predicate buf, :frozen?, 'no inadvertant freeze'
-          assert_equal buf.bytesize, n, 'write wrote expected size'
-          assert_equal s, n, 'syswrite wrote expected size'
+          assert_equal buf.bytesize, n, 'IO#write wrote expected size'
+          assert_equal s, n, 'IO#syswrite wrote expected size'
         end
       end
     end