In a48ea597d0 we fixed settings in private
QtWebEngine windows.
However, this means we also enable local storage for private windows, which was
disabled in QtWebEngine by default:
4ef5831a39 (diff-44ac7d27348388501944f6a8e2e67d8dR207)
It should be safe to enable it, as we get the same behavior as in Chromium, i.e.
a working local storage which entirely lives in RAM.
This also makes those tests work on QtWebKit-NG, presumably because private
browsing for cookies is implemented there.
It also adds a test to at least check whether local storage is isolated from
non-private tabs. I tried writing a test which ensures nothing lands on the hard
disk, but due to QTBUG-52121 this might not happen at all:
https://bugreports.qt.io/browse/QTBUG-52121
