Add expectResumeRejected to note cases where we expect a resumption
handshake to be rejected. (This was previously done by adding a flag,
which is a little less clear.)

Also, save the result of crypto/tls.Conn.ConnectionState() rather than
repeat that a lot.

Change-Id: I963945eda5ce1f3040b655e2441174b918b216b3
Reviewed-on: https://boringssl-review.googlesource.com/4980
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>