tests: Introduce afstest_MyHostAddr

tests: Introduce afstest_MyHostAddr

Several places resolve the local hostname into an IP address.
Consolidate these into a single function (gethostaddr), and add the
function afstest_MyHostAddr, which caches the IP and bails if we can't
resolve our hostname.

Change-Id: I7f71cd136796e4395c639eed8dd8eb19a7b9beec
Reviewed-on: https://gerrit.openafs.org/14802
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>