This aligns our behaviour closer to other browsers, which
_mostly_ consider file scheme URLs as opaque. For test
purposes, allow overriding this behaviour with a commandline
flag.
It turns out that the failing tests actually regressed in:
3e0ea4f62e, which requires more
investigation.
I don't think we would be better off with a revert (and all browsers
already have different behaviour here), so let's just add an in
tree test for now.