Checks that one or more URLs are reachable via HTTP.

Note that the HTTP connectivity can just be verified from the server to the remote URL,
it can still fail if the URL in question is requested by the client, e.g. through an iframe.

Requires curl to present, so ensure to check it before with the following:
<code>EnvironmentCheckSuite::register('check', 'HasFunctionCheck("curl_init")', "Does PHP have CURL support?");</code>