Add a test case for renegotation with False Start enabled.

Historically we had a bug around this and this change implements a test
that we were previously carrying internally.

Change-Id: Id181fedf66b2b385b54131ac91d74a31f86f0205
Reviewed-on: https://boringssl-review.googlesource.com/5380
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>