ipatests: in test_trust.py fix parent class
TestExternalTrustWithRootDomain was inherited from ADTrustSubdomainBase
This caused that external trust was checked two times with subdomain
and was not checked with root domain.
Related to https://pagure.io/freeipa/issue/7889
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>