Pagetests should use the test browser

A number of old pagetests don't use the built-in test browser, instead building their own HTTP headers and what have you and manually parsing responses.

An example of this can be found in pagetests/bugs/xx-bugtask-assignee-widget.txt.

These tests should be refactored to use the test browser, not only to bring them into line with other tests but also (more importantly) to make the tests readable to developers.