ipatests: test_legacy_clients: Change "test group" to "testgroup"
The integration test for legacy clients used incorrectly "test group"
instead of "testgroup" as group used on AD for test purposes. This
is inconsistent with the usage of "testuser".
https://fedorahosted.org/freeipa/ticket/4131
Reviewed-By: Petr Viktorin <pviktori@redhat.com>