Add test for the BIO_s_mem rdwr->rdonly->rdwr use-case

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8649)

(cherry picked from commit 06add280d90de9625e9c18985f376ef8d0419a46)