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, 67 insertions, 0 deletions

diff --git a/actionpack/test/dispatch/system_testing/driver_test.rb b/actionpack/test/dispatch/system_testing/driver_test.rb
index a824ee0c84..2ab8fac666 100644
--- a/actionpack/test/dispatch/system_testing/driver_test.rb
+++ b/actionpack/test/dispatch/system_testing/driver_test.rb
@@ -2,6 +2,7 @@
 
 require "abstract_unit"
 require "action_dispatch/system_testing/driver"
+require "selenium/webdriver"
 
 class DriverTest < ActiveSupport::TestCase
   test "initializing the driver" do
@@ -51,4 +52,70 @@ class DriverTest < ActiveSupport::TestCase
   test "registerable? returns false if driver is rack_test" do
     assert_not ActionDispatch::SystemTesting::Driver.new(:rack_test).send(:registerable?)
   end
+
+  test "define extra capabilities using chrome" do
+    driver_option = nil
+    driver = ActionDispatch::SystemTesting::Driver.new(:selenium, screen_size: [1400, 1400], using: :chrome) do |option|
+      option.add_argument("start-maximized")
+      option.add_emulation(device_name: "iphone 6")
+      option.add_preference(:detach, true)
+
+      driver_option = option
+    end
+    driver.use
+
+    expected = { args: ["start-maximized"], mobileEmulation: { deviceName: "iphone 6" }, prefs: { detach: true } }
+    assert_equal expected, driver_option.as_json
+  end
+
+  test "define extra capabilities using headless_chrome" do
+    driver_option = nil
+    driver = ActionDispatch::SystemTesting::Driver.new(:selenium, screen_size: [1400, 1400], using: :headless_chrome) do |option|
+      option.add_argument("start-maximized")
+      option.add_emulation(device_name: "iphone 6")
+      option.add_preference(:detach, true)
+
+      driver_option = option
+    end
+    driver.use
+
+    expected = { args: ["start-maximized"], mobileEmulation: { deviceName: "iphone 6" }, prefs: { detach: true } }
+    assert_equal expected, driver_option.as_json
+  end
+
+  test "define extra capabilities using firefox" do
+    driver_option = nil
+    driver = ActionDispatch::SystemTesting::Driver.new(:selenium, screen_size: [1400, 1400], using: :firefox) do |option|
+      option.add_preference("browser.startup.homepage", "http://www.seleniumhq.com/")
+      option.add_argument("--host=127.0.0.1")
+
+      driver_option = option
+    end
+    driver.use
+
+    expected = { "moz:firefoxOptions" => { args: ["--host=127.0.0.1"], prefs: { "browser.startup.homepage" => "http://www.seleniumhq.com/" } } }
+    assert_equal expected, driver_option.as_json
+  end
+
+  test "define extra capabilities using headless_firefox" do
+    driver_option = nil
+    driver = ActionDispatch::SystemTesting::Driver.new(:selenium, screen_size: [1400, 1400], using: :headless_firefox) do |option|
+      option.add_preference("browser.startup.homepage", "http://www.seleniumhq.com/")
+      option.add_argument("--host=127.0.0.1")
+
+      driver_option = option
+    end
+    driver.use
+
+    expected = { "moz:firefoxOptions" => { args: ["--host=127.0.0.1"], prefs: { "browser.startup.homepage" => "http://www.seleniumhq.com/" } } }
+    assert_equal expected, driver_option.as_json
+  end
+
+  test "does not define extra capabilities" do
+    driver = ActionDispatch::SystemTesting::Driver.new(:selenium, screen_size: [1400, 1400], using: :firefox)
+
+    assert_nothing_raised do
+      driver.use
+    end
+  end
 end