Make `driven_by` overridable

Sometimes we want to use rack_test partially instead of selenium for test speed:

```ruby
class ApplicationSystemTestCase < ActionDispatch::SystemTestCase
  driven_by :selenium, using: :chrome, screen_size: [1400, 1400], options: {url: "http://chrome:4444/wd/hub"}
end

class WithJavaScriptTest < ApplicationSystemTestCase
end

class WithoutJavaScriptTest < ApplicationSystemTestCase
  driven_by :rack_test
end
```

In the abobe case, `WithoutJavaScriptTest` uses selenium because
`SystemTestCase` calls superclass' driver on `#initialize` (`self.class.superclass.driver.use`).

Using `class_attribute` can handle inherited `driven_by`.

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