Make strftime tests musl compatible

Make strftime tests musl compatible

 * Remove usage of strftime() in favor of date() in cases where
   we are not specifically testing strftime(). We implement
   date() ourselves, and as such are insulated from implementation-
   defined behavior.
 * Add skipif for broken strftime() %Z support. We have decided
   not to work around the issue for musl using manual expansion,
   as people should not be using this function anyway, and it is
   slated for future deprecation.
 * Don't test strftime() with invalid format specifier. The
   behavior is implementation-dependent.