so makes the full suite brittle.  Cleaned out all the places where unit
tests change sys.path.  (Use the PYTHONPATH environment variable instead.)

Also brought SearchIndex tests up to date, which I didn't really need to do
since SearchIndex is deprecated, but I did it as part of figuring out the
failed Interface tests, so I might as well check in my work. ;-)  One of the
test modules in SearchIndex forgets to define a global named "Dummy",
actually, so all the tests were failing before this checkin anyway.