Fix memory leak in session cache test

When we are using the internal cache we have to make a copy of the
session before removing it from the parent context's cache, since
we want our copy to still be resumable.  However, SSL_CTX_remove_session()
just detaches the session from the SSL_CTX; it does not free the session.
So, we must call SSL_SESSION_free() ourselves before overwriting the
variable that we dup'd from.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4126)