protover_get_supported_protocols() should include Link=5 as of 0.3.1.1-alpha

Fortunately, link version negotiation does not rely on protover. But I added some unit tests to make sure we check for new Link versions.

This isn't easy, because most protovers don't expose supported versions in headers. And those that do, don't expose a list or a maximum version. I opened legacy/trac#25069 (moved) to refactor the code so it's easier to test.