Only run tests that require libidn when we have libidn available

This should eliminate the need for https://gitweb.gentoo.org/repo/gentoo.git/tree/net-im/pidgin/files/pidgin-2.13.0-disable-one-jid-test.patch

Testing Done:
Compiled with and with out `--disable-idn`, also added a `#warning` during testing to verify if the code was being include properly.

Reviewed at https://reviews.imfreedom.org/r/922/