allow testing even if name->addr->name mapping doesn't work

allow testing even if name->addr->name mapping doesn't work

Many of the tests are set up to fail if the local hostname can't be
mapped to an address and back to a name again.  If the name results in
an address, and we can get a fully-qualified name or something that
looks like it, though, we should be able to just go ahead and run some
tests.

This is also closer to the current behavior of sname_to_principal when
reverse DNS is enabled.

ticket: 6590

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@23439 dc483132-0cff-0310-8789-dd5450dbe970