test_extesion is testing too many features at once and is hard to
navigate. Let's split each chunk apart for more clarity.

The implementation of OpenSSL::X509::Certificate#crl_uris makes the
assumption that each DistributionPoint in the CRL distribution points
extension contains a single general name of type URI. This is not
guaranteed by RFC 5280. A DistributionPoint may contain zero or more
than one URIs.

Let's include all URIs found in the extension. If only non-URI pointers
are found, return nil.

Fixes: ruby#775