Unfortuntately, the snapshot of test/unit included in Ruby 1.8.2 is broken for use with Rake. Steps are being made to remedy the situation, but in the mean while, you can apply the fix needed to the Ruby source itself. It’s a two-line change, but pretty important two lines.
In order to apply this fix, you need to:
(or where ever you have Ruby installed) and do:
patch -p0 < /path/to/fix/dir.rb.cleanup.patch
Hopefully a more permanent solution can be found in Rake or Ruby shortly.