tests: add test for server install with --no-dnssec-validation
Add 2 tests related to the checks performed by ipa-server-install
when --forwarder is specified:
- if the forwarder is not reachable and we require dnssec validation,
the installer must refuse to go on and exit on error.
- if the forwarder is not reachable but --no-dnssec-validation is
provided, the installer must continue.
Related to https://pagure.io/freeipa/issue/7666
Reviewed-By: Tibor Dudlak <tdudlak@redhat.com>