Add regression tests for AD machine account keytab generation

PR: https://github.com/freenas/freenas/pull/5291

When TrueNAS joins an AD domain, a kerberos keytab must be
auto-generated for the AD machine account. These two tests verify
that our current system keytab contains entries, and verifies
that these entries are regenerated if the are removed, and the
kerberos service is stopped and restarted. Expected behavior
after the kerberos start is that we have keytab entries and
a valid kerberos ticket.

Related to NAS-106963