Consider sanitising usernames and passwords used for the amp-http proxy

[brendonj]

As of aeea62a the amp-http test reports the proxy server used (if any), which can possibly leak sensitive information. Consider removing usernames and passwords before sending the results onto the network. Could the proxy address also be considered sensitive?

The proxy server is not currently a stream descriptor in nntsc, but probably should be so that tests using a proxy can be identified and more easily compared with those not using a proxy. This means something needs to be reported about proxy usage (maybe a boolean flag is good enough?).