Add a blackbox tests for id & getent to test domain@realm type credentials

Add a blackbox tests for id & getent to test domain@realm type credentials

Using domain@realm credentials has been problematic when
global conf setting "winbind use default domain" is enabled, this patch
creates a new s4member_dflt_domain environment (where
"winbind use default domain" is enabled) and runs getent & id against the
normal s4member & and new s4member_dflt_domain environments

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12298

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>