Increase OutputPage test coverage to >45%

Increase OutputPage test coverage to >45%

Also stop returning a value from OutputPage::adaptCdnTTL().  There are
no users and the value doesn't seem very helpful: one would probably
expect it to return the new value of mCdnMaxage, but instead it
returns the new value of mCdnMaxageLimit. Better to have no return
value than one that's easily misunderstood (*and* which nobody uses).

Change-Id: Ia9dab86923b839334eab9f6fde17c4aed52130ec