ipatests: fix TestTrust::test_subordinate_suffix
The test test_subordinate_suffix is failing when configuring the DNS
for the trust, because the dnsforwardzone already exists. It was
configured during the previous test for nonposix trust.
At the end of the tests for nonposix trust, unconfigure the DNS
and the trust before calling the subordinate_suffix test, and add
a test cleaning up subordinate_suffix test.
Fixes: https://pagure.io/freeipa/issue/8601
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>