Implement a way to add browser capabilities:

* There is currently no way to define specific browser capabilities since our SystemTest driver override the `option` key [Ref](https://github.com/rails/rails/blob/a07d0680787ced3c04b362fa7a238c918211ac70/actionpack/lib/action_dispatch/system_testing/driver.rb#L35)
  This option key is used internally by selenium to add custom capabilities on the browser.

  Depending on the Browser, some option are allowed to be passed inside a hash, the driver takes care of setting whatever you passed on the driver option. An example [here](https://github.com/rails/rails/blob/a07d0680787ced3c04b362fa7a238c918211ac70/actionpack/lib/action_dispatch/system_testing/driver.rb#L35) where you are allowed to pass args such as `--no-sandbox` etc
  However this behavior was only meant for backward compatibility and as you can see it's deprecated.
  The non-deprecated behavior is to create a `<Driver>::Option` object containing all the capabilities we want. This is what we [currently do](https://github.com/rails/rails/blob/a07d0680787ced3c04b362fa7a238c918211ac70/actionpack/lib/action_dispatch/system_testing/browser.rb#L34-L36) when chrome or firefox are in headless mode.

  This PR allows to pass a block when calling `driven_by`, the block will be pased a `<Driver>::Option` instance. You can modify this object the way you want by adding any capabilities. The option object will be then passed to selenium.

  ```ruby
    driven_by :selenium, using: :chrome do |driver_option|
      driver_option.add_argument('--no-sandbox')
      driver_option.add_emulation(device: 'iphone 4')
    end
  ```

1 files changed, 8 insertions, 1 deletions

diff --git a/actionpack/lib/action_dispatch/system_testing/driver.rb b/actionpack/lib/action_dispatch/system_testing/driver.rb
index 5252ff6746..87b72a2048 100644
--- a/actionpack/lib/action_dispatch/system_testing/driver.rb
+++ b/actionpack/lib/action_dispatch/system_testing/driver.rb
@@ -3,11 +3,12 @@
 module ActionDispatch
   module SystemTesting
     class Driver # :nodoc:
-      def initialize(name, **options)
+      def initialize(name, **options, &desired_capabilities)
         @name = name
         @browser = Browser.new(options[:using])
         @screen_size = options[:screen_size]
         @options = options[:options]
+        @desired_capabilities = desired_capabilities
       end
 
       def use
@@ -22,6 +23,8 @@ module ActionDispatch
         end
 
         def register
+          define_browser_capabilities(@browser.driver_option)
+
           Capybara.register_driver @name do |app|
             case @name
             when :selenium then register_selenium(app)
@@ -31,6 +34,10 @@ module ActionDispatch
           end
         end
 
+        def define_browser_capabilities(driver_option)
+          @desired_capabilities.call(driver_option) if @desired_capabilities
+        end
+
         def browser_options
           @options.merge(options: @browser.options).compact
         end