Silence warning from MySQL::Error object in test

When running Active Record MySQL test, this warning is printed in the
console:

    warning: instance variable errno not initialized

It turns out that this is a warning from `mysql` gem in MySQL::Error
object. However, since the `mysql` gem is no longer maintained, and
there won't be a newer version, it make sense for us to just silence
this warning to make the output cleaner.

activerecord/test/cases/disconnected_test.rb

@@ -21,7 +21,9 @@ def setup
       @connection.execute "SELECT count(*) from products"
       @connection.disconnect!
       assert_raises(ActiveRecord::StatementInvalid) do
-        @connection.execute "SELECT count(*) from products"
+        silence_warnings do
+          @connection.execute "SELECT count(*) from products"
+        end
       end
     end
   end