Skip test properly if we can't change TZ

On Windows, with the test modules bundled with 5.14.2, it was printing
the skip message but exiting 255, causing cpan to refuse to install it.
Handle unimplemented POSIX::tzset

Should fix
http://www.cpantesters.org/cpan/report/3b1b9d2d-6d69-1014-a93d-93423e5e3ebd
Merge branch 'skipping' of https://github.com/madsen/Time-modules into madsen-skipping