Dotted FQDN tests may fail or succeed depending on system DNS configuration

[mgorny]

Related to #1475, #1648.

I've recently spent some time debugging test failures reported by Gentoo users, and came to the conclusion that resolving localhost. may work or not depending on the system resolver configuration. I've been able to make it work reliably by listing localhost. in /etc/hosts explicitly or by using nss_myhostname. It also may work when external DNS is used and works, though I have no clue why glibc makes it work like that. On the other hand, if the system can't connect to the DNS resolver, it fails.

Since this test is unreliable, could you please disable (skip) it by default and enable only on known-good travis infra? I would supply a patch but I don't really know what values TRAVIS_INFRA can have.

[sethmlarson]

Yep I definitely run into this issue as well. Maybe we need to skip the test if we can't resolve localhost., change the decorator to run that DNS lookup and on failure just skip.