Add test coverage for session resumption with tickets.

The shim is now passed two file descriptors. In a session resumption test, the
second is used in an abbreviated handshake immediately after the first.

Change-Id: I1f348c05f1a8ff2881fb46fc9e869696f74071c6
Reviewed-on: https://boringssl-review.googlesource.com/1291
Reviewed-by: Adam Langley <agl@google.com>