crypto/tls: update how we create testing scripts.

crypto/tls: update how we create testing scripts.

crypto/tls is tested, in part, by replaying recorded TLS connections
and checking that the bytes sent by the Go code haven't changed.

Previously we used GnuTLS's debug output and extracted the bytes of
the TLS connection using a Python script. That wasn't great, and I
think GnuTLS removed that level of debugging in a more current
release.

This change records the connection with Go code and adds a test for
ECDHE-AES clients generating using this method.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5988048