libstdc++: Uncomment denorm_min test

As r13-3609-g6d9dbdf51f9afe8 has been committed, we can now enable
even the denorm_min test.

2022-11-08  Jakub Jelinek  <jakub@redhat.com>

	* testsuite/20_util/to_chars/float128_c++23.cc (test): Uncomment
	denorm_min test.