Make test_url script more usable

What does this Merge Request do and why?

This MR moves the test_url script out from support/ci into just support to indicate it's not only usable for CI but a user can also run:

$ gdk start ; support/test_url
--snip--
=> GitLab will be available at http://127.0.0.1:3000 shortly.
.......................
http://127.0.0.1:3000/users/sign_in is up! Took 59 second(s).

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.