Pass the test reporter by reference

This prevents the array from being dumped as a DRbObject so we can reduce
communication with the server.

In DRb, if `Marshal.dump` fails, `Marshal.dump` is executed again after
converting the object to `DRbObject`. This also possible to reduce the
execution of `Marshal.dump` by converting to a format that can be
marshalized in advance using `DRbObject`.
This is the same approach to Action Pack's parallel test. Ref: 5751b7e

activesupport/lib/active_support/testing/parallelization.rb

@@ -23,6 +23,7 @@ def record(reporter, result)
         end
 
         def <<(o)
+          o[2] = DRbObject.new(o[2]) if o
           @queue << o
         end