Since the JSCore used by WebKit 602.1 doesn't fully support Proxy and I
can't think of a way to provide isolation otherwise just revert to the
old behaviour in that case. I am checking for the specific WebKit
version because I'm pretty sure that version just happened to be
released when Proxy support was only partially done, any later release
will presumably have a newer JSCore where it works.
There I changed the indentation of a block in the jinja template which
will have inflated the diff.
I added mocking of `objects.backend` to the `webview` and
`webenginewebview` fixtures, I am pretty sure they are mutually
exclusive so don't expect any issues from that.
Because of the feature detection being at template compile time I had to
tweak the test setup to be done via a fixture instead of the setupClass
functionality that I was using before.
