crypto/tls: tidy up a little and add test.

crypto/tls: tidy up a little and add test.

This is a follow on to 28f33b4a which removes one of the boolean flags
and adds a test for the key-driven cipher selection.

Change-Id: If2a400de807eb19110352912a9f467491cc8986c
Reviewed-on: https://go-review.googlesource.com/8428
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>
Reviewed-by: Jacob Haven <jacob@cloudflare.com>