test: remove uses of common.PORT in test-tls-client tests

Change common.PORT to '0' to avoid the possibility of getting EADDRINUSE error
if another test in 'parallel' uses port '0' at the same time.

PR-URL: https://github.com/nodejs/node/pull/12461
Ref: https://github.com/nodejs/node/issues/12376


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>