TLSProxy: When in debug mode, show the exact subprocess commands

When you want to debug a test that goes wrong, it's useful to know
exactly what subprocess commands are run.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3342)