Bug 851320 - private-browsing-supported/test-page-mod fails

Replaced `about:buildconfig` with a more unique URL. The failure is most likely related to another test that open the same URL but with a different private setting. We already had a similar issue, fixed in the same way.
Merge pull request #859 from ZER0/page-mod-test/851320

fix Bug 851320 - private-browsing-supported/test-page-mod fails r=@mossop