Clear gem paths for each test

So that rubygems can find the bundled rake.

Author: nobu

tool/lib/test/unit/parallel.rb

@@ -87,6 +87,8 @@ def _run_suite(suite, type) # :nodoc:
         $stdout = orig_stdout if orig_stdout
         o.close if o && !o.closed?
         i.close if i && !i.closed?
+
+        Gem.clear_paths if defined?(Gem)
       end
 
       def run(args = []) # :nodoc: