ipatests: test_trust: Add test to cover lookup of trusdomains
Adds an integration tests that checks that all trustdomains are
able to be found by trustdomain-find command right after the
trust has been established.
Also moves some code to allow easier adding common test cases for
both POSIX and non-POSIX test classes.
https://fedorahosted.org/freeipa/ticket/4208
Reviewed-By: Petr Viktorin <pviktori@redhat.com>