submodule--helper: move "check-name" to a test-tool

submodule--helper: move "check-name" to a test-tool

Move the "check-name" helper to a test-tool, since
a6226fd772b (submodule--helper: convert the bulk of cmd_add() to C,
2021-08-10) it has only been used by this test, not git-submodule.sh.

As noted with its introduction in 0383bbb9015 (submodule-config:
verify submodule names as paths, 2018-04-30) the intent of
t7450-bad-git-dotfiles.sh has always been to unit test the
check_submodule_name() function.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>