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
Merge branch '4208_trustdomain_test' into test