Add tests for client-initiated renegotiation.

These'll get removed once most of renego support is gone, but this is to prove
removing the warning alert from the previous commit still prevents legacy
renegotiations.

Change-Id: I7d9d95e1d4c5d23d3b6d170938a5499a65f2d5ea
Reviewed-on: https://boringssl-review.googlesource.com/2236
Reviewed-by: Adam Langley <agl@google.com>