This commit adds some session API tests, and in particular tests the
modified behaviour of SSL_set_session() introduced in the last commit. To
do this I have factored out some common code from the asynciotest into a
new ssltestlib.c file. I've also renamed getsettest to sslapitest as this
more closely matches what it now is!
Reviewed-by: NRich Salz <rsalz@openssl.org>

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: NAndy Polyakov <appro@openssl.org>