Convert test suite to run headless (#658)

* Register headless Chrome driver for tests

- Added DriverRegistration.register_selenium_chrome_headless call to ensure
  the headless driver is properly initialized before tests run
- Tests already configured to use selenium_chrome_headless but driver
  wasn't being registered, causing browser window to flash
- No browser window will appear during test runs now

Author: justin808

spec/rails_helper.rb

@@ -42,6 +42,8 @@
   # Ensure that if we are running js tests, we are using latest webpack assets
   # This will use the defaults of :js and :server_rendering meta tags
   ReactOnRails::TestHelper.configure_rspec_to_compile_assets(config)
+  # Register the headless Chrome driver
+  DriverRegistration.register_selenium_chrome_headless
 
   # Remove this line if you're not using ActiveRecord or ActiveRecord fixtures
   config.fixture_path = Rails.root.join("spec/fixtures")