[plt-scheme] planet: version collision on schemeunit.plt

I tried the new Jacob added for tracking these things down. It was not
useful (but it also couldn't have been, in this case), so I looked
into things a little bit more.

The schemeunit 2.6 tool seems broken. It always seems to be requiring
itself in to the user's namespace!

              (parameterize ((current-namespace user-ns))
                ((dynamic-require UI-MODULE-SPEC 'initialize)
                 has-backtrace?
                 show-backtrace
                 show-source))))

I'm not sure why it does that, but that would be more like a language
extension and probably not appropriate.

I hope the maintainers of Schemeunit can remove this. If someone can
explain why it does that, maybe I can suggest an alternative.

Robby