Use FrozenError instead of RuntimeError in frozen object modification test

test/t/string.rb

@@ -711,7 +711,7 @@ assert('String#freeze') do
   str = "hello"
   str.freeze
 
-  assert_raise(RuntimeError) { str.upcase! }
+  assert_raise(FrozenError) { str.upcase! }
 end
 
 assert('String literal concatenation') do