Don't attempt to use DNS when no nameserver is available.

* src/server/testsuite/Makefile.am (site.exp): Set use_dns to yes
or no depending on if adnshost can resolve 127.0.0.1 or not.
* src/server/testsuite/config/unix.exp (lyskomd_start): Set the
"Use DNS" configuration depending on the use_dns setting from
site.exp.