test_smbclient_s3.sh: Use correct separator in "list with backup privilege" test

test_smbclient_s3.sh: Use correct separator in "list with backup privilege" test

Samba selftest uses the forward slash as winbind separator and in the
USERNAME passed to the test. "net sam rights" expect the backslash. Map
the separator used in selftest to a backslash to avoid creating an
incorrect username DOMAIN\DOMAIN/USERNAME.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13312

Signed-off-by: Christof Schmitt <cs@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
(cherry picked from commit 6f07afad07d9c670a00d9d314a8134efdda5e424)