ipatests: fix collection of tests from test_trust suite
    
    Commit 969b4c87 which added test_extdom_plugin case also mistakenly
    renamed class from TestTrust to BaseTestTrust. As pytest collects tests
    only from classes staring with "Test", no tests are now executed in
    test_trust module.
    Fixing by partially reverting said commit
    
    Reviewed-By: Stanislav Levin <slev@altlinux.org>