Add a test for the UI API

The best way to test the UI interface is currently by using an openssl
command that uses password_callback.  The only one that does this is
'genrsa'.
Since password_callback uses a UI method derived from UI_OpenSSL(), it
ensures that one gets tested well enough as well.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2040)