* Treat 304 (Not Modified) requests as valid.
* Add tests for 301-to-200 links, 301-to-404 links, and 500 links.
This helps to test redirections and the previously-added
response.status() checking for non-success status codes in check_url().
* Make names for HTTP mock paths unique, to avoid weird behavior. It
seems like mocks with the same path can potentially bleed between
tests, so you may end up with an unexpected response which causes the
test to sometimes pass and sometimes fail.
* Fix Clippy warnings about String::from(format!()).
Certain tests involving HTTP requests were sometimes hanging
indefinitely, so this uses Mockito for HTTP mocking. This seemingly
resolves the issue and makes these tests more reliable.
The existing can_fail_404_links test has been renamed to
can_fail_unresolved_links, to represent what actually occurs in the
test. The can_fail_404_links test now deals with a proper 404
response.
Just to be clear, the check_site test in the site component will
still create outgoing HTTP requests (due to the URLs used in the
test_site), so this commit only uses HTTP mocking where possible.