only test t_locate_kdc if known-good DNS name is present

only test t_locate_kdc if known-good DNS name is present

Running "make check" while offline or on a firewalled network may
result in failure in lib/krb5/os because the invocation of
t_locate_kdc requires that the DNS servers for ATHENA.MIT.EDU be
reachable.  Autodetect DNS utilities "dig" and "nslookup", and use
them to check for existence of the known-good DNS name.  Also
parameterize the test so that the known-good DNS name can be
overridden on the make command line.

ticket: 6744
target_version: 1.8.3
tags: pullup

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@24138 dc483132-0cff-0310-8789-dd5450dbe970