testserver does not respect EDNS buffer size

Testserver currently hardcodes EDNS version 0 buffer size 4096 and ignores EDNS parameters sent by client. This leads to situation where testserver might send back oversized UDP response which is later refused by client.

We need to discuss what to do with that. Current behavior breaks tests if resolver under test is using different value than 4096 B.

Options: a) Add ability to specify alternative ENTRY when first matching ENTRY does not fit into EDNS buffer. This is probably too verbose because in vast majority of cases we want to send back empty response with TC bit and let client fall back to TCP.

b) Detect that current message size if bigger than min(client, Deckard) values and automatically generate response with TC=1. Do we need ability to override this automatic behavior?

TODO: Check if queries over TCP actually work with answers bigger than EDNS buffer size.