submodule--helper: move "resolve-relative-url-test" to a test-tool

submodule--helper: move "resolve-relative-url-test" to a test-tool

As its name suggests the "resolve-relative-url-test" has never been
used outside of the test suite, see 63e95beb085 (submodule: port
resolve_relative_url from shell to C, 2016-04-15) for its original
addition.

Perhaps it would make sense to drop this code entirely, as we feel
that we've got enough indirect test coverage, but let's leave that
question to a possible follow-up change. For now let's keep the test
coverage this gives us.

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>