gnutls-serv: improvements in UDP server

This modifies the server to deinitialize the session after use (avoiding leaks), and to only send the hello verify request when a client hello is seen.

This also adds a basic unit test of gnutls-serv with the --udp option.

Checklist

• Code modified for feature
• Test suite updated with functionality tests (general)

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code