It's a fairly small set of users who use this feature, so I'd prefer we didn't directly pollute `ExampleGroup` with this.  Instead, what do you think about doing something like this?

``` ruby
module StdlibAssertionsAdapter
  include Minitest::Assertions # or Test::Unit::Assertions

  attr_writer :assertions
  def assertions
    @assertions ||= 0
  end
end
```

...and then make the `expect_with :stdlib` config include this module.