Unregress update isolation tests. Rethink offset fallback.

Now, when there is no existing item on page (when offset is past end of page),
we invoke _bt_check_unique with a special "Don't look at heap" argument for our
ctid-lacking IndexTuple. This doesn't actually fix the page-split bug, but it's
very likely a prerequisite to doing so.

The update isolation tests have been broken for a few days now, but are fixed
in this commit.