Testing safe renegotiation is relatively complex, here is a summary of
what we test and what how we believe it should work.

srn0.c:

 This tests that the safe renegotiation extension is negotiated
 properly by default on initial connections and on rehandshaked
 connections.  Consequently, it also verifies that rehandshaked
 connections work with the extension enabled.

srn4.c:

 This tests that clients without support for safe renegotiation is
 able to handshake properly against a default GnuTLS server, but
 that rehandshake will be refused by the server.

srn5.c:
 This tests that clients with support for safe renegotiation is able
 to handshake properly against a server with no support for safe
 renegotiation, but that the client will not be able to complete a
 rehandshake.
