Proposed fix for test failures due to long socket paths

[David Bremner]

Amadeusz Żołnowski found a bug in the test suite that causes gpg
failures when the path of the test directory is sufficiently long.
His current solution in Gentoo is to move the sockets into /tmp. It
seems cleaner to enable gnupg's built in /run based short pathed
sockets.  As far as I know the gpgconf option used here is available
in gnupg 2.1.13 and later. Amadeusz reported that the problem was
"fixed" in Gentoo by downgrading gnupg to 2.1.15. So while I'm not
100% sure, it seems like a fix for very recent gnupg is all that is
required. Testing of these patches with older gpg would be
particularly welcome.