NSS/TESTS: Improve setup/teardown for subdomains tests
    
    This patch basically makes the getgrnam_members_subdom(),
    getgrnam_mix_dom(), getgrnam_mix_dom_fqdn() and getgrnam_mix_subdom()
    more independent of each other.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/3001
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>