test/ssl_test: give up if both client and server wait on read

In some cases, both client and server end of the test can end up in
SSL_ERROR_WANT_READ and never get out of it, making the test spin.
Detect it and give up instead of waiting endlessly.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2096)