Add tests for case insensitive tree iterator

This adds a test case for ci tree iteration when there is a name
conflict.  This points out a behavior quirk in the current version
that I'd like to fix - namely, all tree entries get mapped to one
version of the case pattern in the ci code - i.e. even if you have
A/1.txt and a/2.txt, both will be reported as a/1.txt and a/2.txt
because we only copy the name of a file at a given frame once. It
would be nice to fix this, but I'm worried about how complex that
is if you get a/B/c/1.txt and A/b/C/2.txt.  It may require a walk
up the frames whenever you advance to the next item in a blended
equivalence class.
Use correct case path in icase tree iterator

If there are case-ambiguities in the path of a case insensitive
tree iterator, it will now rewrite the entire path when it gives
the path name to an entry, so a tree with "A/b/C/d.txt" and
"a/B/c/E.txt" will give the true full paths (instead of case-
folding them both to "A/B/C/d.txt" or "a/b/c/E.txt" or something
like that.