TEST: Use right domain name in negcache test
Directory tests_ncache was not removed after negcache test,
because sysdb cache had different name and was not removed in the function
test_dom_suite_cleanup.
[sssd] [test_dom_suite_cleanup] (0x0020):
Could not delete the test dir (39) (Directory not empty)
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>