TLSProxy/Proxy.pm: handle "impossible" failure to connect to s_server.

The failure is "impossible", because we have confirmation that s_server
listens, yet Mac OS X fails to connect. This avoids 10 minutes timeout
on Travis CI.

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5907)