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`.

1 files changed, 8 insertions, 0 deletions

diff --git a/actionpack/test/dispatch/system_testing/system_test_case_test.rb b/actionpack/test/dispatch/system_testing/system_test_case_test.rb
index 1a9421c098..33d98f924f 100644
--- a/actionpack/test/dispatch/system_testing/system_test_case_test.rb
+++ b/actionpack/test/dispatch/system_testing/system_test_case_test.rb
@@ -6,6 +6,14 @@ class SetDriverToRackTestTest < DrivenByRackTest
   end
 end
 
+class OverrideSeleniumSubclassToRackTestTest < DrivenBySeleniumWithChrome
+  driven_by :rack_test
+
+  test "uses rack_test" do
+    assert_equal :rack_test, Capybara.current_driver
+  end
+end
+
 class SetDriverToSeleniumTest < DrivenBySeleniumWithChrome
   test "uses selenium" do
     assert_equal :selenium, Capybara.current_driver