testutil: Move printing function declarations to "internal" header

These functions aren't meant to be used directly by the test programs,
reflect that by making the declarations a little harder to reach, but
still available enough if there's a need to override them.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3345)