TLSProxy/Proxy.pm: preclude output intermix.

s_server -rev emits info output on stderr, i.e. unbufferred, which
risks intermixing with output from TLSProxy itself on non-line
boundaries, which in turn is confusing to TAP parser.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5975)