Remove excessive check of message

The order of "unexpected" and "missing" keyword argument errors at
`Data#initialize` had not been defined strictly.

[Bug #21844](https://bugs.ruby-lang.org/issues/21844)
ruby/ruby#15910

Author: nobu

test/psych/test_data.rb

@@ -83,12 +83,11 @@ def test_members_must_be_identical
 
       # completely different members
       TestData.send :remove_const, :D
-      TestData.const_set :D, Data.define(:foo, :bar)
+      TestData.const_set :D, Data.define(:a, :c)
       e = assert_raise(ArgumentError) { Psych.unsafe_load d }
-      assert_equal 'unknown keywords: :a, :b', e.message
+      assert_include e.message, 'keyword:'
     ensure
       TestData.send :remove_const, :D
     end
   end
 end
-