t7450: test submodule urls

t7450: test submodule urls

Add tests to 't7450-bad-git-dotfiles.sh' to check the validity of different
submodule URLs. To verify this directly (without setting up test
repositories & submodules), add a 'check-url' subcommand to 'test-tool
submodule' that calls 'check_submodule_url' in the same way that
'check-name' calls 'check_submodule_name'.

Add two tests to separately address cases where the URL check correctly
filters out invalid URLs and cases where the check misses invalid URLs. Mark
the latter ("url check misses invalid cases") with 'test_expect_failure' to
indicate that this is currently broken, which will be fixed in the next step.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>