ipatests: rewrite test for requests routing to subordinate suffixes
The original test had some issues:
* it was doing many actions not related to the tested issue which obscured
actual test scenario
* subordinate suffix was hard coded in the test which prevented the test
from checking original issue in case AD domain name did not match this
hard coded value
* Invocation of commands on AD controller was failing in some environments
Other improvements:
* added docstring with test details
* added guard assertions for test preliminary conditions
Related to https://pagure.io/freeipa/issue/8554
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>