Make test number t7406- unique

We skip t7407 because a patch series is cooking that uses is.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/t/t7406-submodule-reference.sh b/t/t7408-submodule-reference.sh
similarity index 100%
rename from t/t7406-submodule-reference.sh
rename to t/t7408-submodule-reference.sh