Add a DTLSv1_listen() test

Add a DTLSv1_listen() test

Add a test to ensure that a connection started via DTLSv1_listen() can
be completed through to handshake success. Previous DTLSv1_listen()
testing only tested the function itself and did not confirm that a
connection can actually be achieved using it.

This is important to test some codepaths being affected by the record layer
refactor.

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18132)