Add tests for session-ID-based resumption.

This implements session IDs in client and server in runner.go.

Change-Id: I26655f996b7b44c7eb56340ef6a415d3f2ac3503
Reviewed-on: https://boringssl-review.googlesource.com/2350
Reviewed-by: Adam Langley <agl@google.com>
4 files changed