tests: Improve failure mode for unresolvable hostname

In the case of a host where gethostbyname is unable to resolve
the hostname, afstest_BuildTestConfig() may return NULL which
can cause several tests to crash.

Add a common function to look out for this condition and use it where
appropriate.  When it occurs, the current module is skipped and
the user gets an error message that indicates the configuration
problem.

Change-Id: I7216876eb2424368f415e5759e2b95009ad055b2
Reviewed-on: http://gerrit.openafs.org/9120
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>