Apply code review suggestion from CL 46715.

The block is doing more than just checking len(r.certs) == len(tc.cns).
It also verifies that certificate common names match.

Change-Id: I28d6926a5da48bd8f6c80aa5e5a1ed6d4990f845
Reviewed-on: https://go-review.googlesource.com/47132Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>