keepalive-test: prevent false positives, filter a little more

From the comments at https://github.com/pagespeed/ngx_pagespeed/pull/425,
on Chai's machine curl's output seems slightly different when it
reports it is trying to connect.
Merge pull request #431 from pagespeed/oschaaf-fix-keepalive-test

keepalive-test: prevent false positives, filter a little more