Passing multiple URLs via --specific-url does not work properly

[stevejalim]

Running something like
/bin/run_checks.py --specific-url http://localhost:8000/en-US --allowlist=data/development-allowlist.yaml --specific-url https://mozilla.org/en-US/ works as far as scanning goes, but it only outputs a results dump for the first specified URL.

While this option is not used in production, it would be good to either support multiple --specific-url calls end-to-end, or to at least limit it to a single use of the option

[stevejalim]

Note: this was done deliberately, and has an awkward side effect (see above). Better might be to just verify that all specific URLs are on the same domain, and if not, stop/fail