ipatests: Remove deprecated yield_fixture
'yield_fixture' is deprecated since Pytest3 [0].
FreeIPA requires at least 3.9.1. So, it can be safely removed.
[0]: https://docs.pytest.org/en/latest/yieldfixture.html
Fixes: https://pagure.io/freeipa/issue/8101
Signed-off-by: Stanislav Levin <slev@altlinux.org>
Reviewed-By: Sergey Orlov <sorlov@redhat.com>