t7406-submodule-update: add missing &&

322bb6e (2011 Aug 11) introduced a new subshell at the end of a test
case but omitted a '&&' to join the two; fix this.

Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>