tree-conflicts tests pass to leniently

We are still expecting stuff in the update, switch and merge tree-conflicts
tests to pass that is obviously not correct, and we know it. We want a
tree-conflict to appear on every D1 node in those tests, and some still
don't show it because we can't compare an arbitrary repos URL_at_REV against
the local working copy with mods (aka diff --summarize). We're just playing
along and PASSing the nonsense current behaviour.

This was convenient in the developing phase, but now that we're about to
branch 1.6, it would be good to XFAIL these tests to clearly mark spec
violations. Good?

~Neels