Add size method to ParallelEach

When I run `PARALLEL=true make test-all TESTS=test/minitest/test_minitest_unit.rb` on trunk ruby, I get this exception:

```
/Users/btaitelb/programming/ruby/ruby/lib/test/unit.rb:707:in `block in _prepare_run': undefined method `size' for #<ParallelEach:0x007f873398ea38> (NoMethodError)
	from /Users/btaitelb/programming/ruby/ruby/lib/test/unit.rb:707:in `each'
	from /Users/btaitelb/programming/ruby/ruby/lib/test/unit.rb:707:in `inject'
	from /Users/btaitelb/programming/ruby/ruby/lib/test/unit.rb:707:in `_prepare_run'
	from /Users/btaitelb/programming/ruby/ruby/lib/test/unit.rb:648:in `_run_suites'
	from /Users/btaitelb/programming/ruby/ruby/lib/minitest/unit.rb:867:in `_run_anything'
	from /Users/btaitelb/programming/ruby/ruby/lib/minitest/unit.rb:1060:in `run_tests'
	from /Users/btaitelb/programming/ruby/ruby/lib/minitest/unit.rb:1047:in `block in _run'
	from /Users/btaitelb/programming/ruby/ruby/lib/minitest/unit.rb:1046:in `each'
	from /Users/btaitelb/programming/ruby/ruby/lib/minitest/unit.rb:1046:in `_run'
	from /Users/btaitelb/programming/ruby/ruby/lib/minitest/unit.rb:1035:in `run'
	from /Users/btaitelb/programming/ruby/ruby/lib/test/unit.rb:21:in `run'
	from /Users/btaitelb/programming/ruby/ruby/lib/test/unit.rb:774:in `run'
	from /Users/btaitelb/programming/ruby/ruby/lib/test/unit.rb:834:in `run'
	from /Users/btaitelb/programming/ruby/ruby/lib/test/unit.rb:838:in `run'
	from ./test/runner.rb:17:in `<main>'
make: *** [yes-test-all] Error 1
```

Test::Unit calls #size on the ParallelEach.