Stop checking exception message

This seems unnecessary given we look for the exception type.

The reason I'm doing this is the message changed in Ruby 3.4.

Author: horgh

test/test_reader.rb

@@ -319,7 +319,6 @@ def test_metadata_unknown_attribute
     e = assert_raises NoMethodError do
       reader.metadata.what
     end
-    assert(e.message.include?('undefined method `what\''))
     reader.close
   end