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.