Add an async io test

Add an async io test

This adds an async IO test. There are two test runs. The first one does
a normal handshake with lots of async IO events. The second one does the
same but this time breaks up all the written records into multiple records
of one byte in length. We do this all the way up until the CCS.

Reviewed-by: Andy Polyakov <appro@openssl.org>