Change inheritance structure to match the one from Rails 5 more

In the future, ApplicationSystemTestCase should inherit from
ActionDispatch::SystemTestCase and JavaScriptApplicationSystemTestCase
could possibly be removed if the `driven_by` method works as it is
promised in Rails documentation:

http://api.rubyonrails.org/v5.2/classes/ActionDispatch/SystemTestCase.html

Consider introducing another class between
ActionDispatch::IntegrationTest and other items inheriting from it, as
the general Rails practice seems to have `ApplicationIntegrationTest`,
as we do have `ApplicationRecord` and `ApplicationController`.

test/system/registrar/sign_in_test.rb

@@ -0,0 +1,35 @@
+require 'test_helper'
+
+class RegistrarAreaSignInTest < JavaScriptApplicationSystemTestCase
+  def setup
+    super
+    WebMock.allow_net_connect!
+
+    @user = users(:api_bestnames)
+    @user.identity_code = '1234'
+    @user.save
+  end
+
+  def test_mobile_id_sign_in_page
+    mock_client = Minitest::Mock.new
+    mock_client.expect(:authenticate,
+                       OpenStruct.new(user_id_code: '1234', challenge_id: '1234'),
+                       [{ phone: "+3721234",
+                         message_to_display: "Authenticating",
+                         service_name: "Testimine" }])
+    mock_client.expect(:session_code, 1234)
+
+    Digidoc::Client.stub(:new, mock_client) do
+      visit registrar_login_path
+
+      click_on 'login-with-mobile-id-btn'
+
+      fill_in 'user[phone]', with: '1234'
+      click_button 'Login'
+
+      flash_message = page.find('div.bg-success')
+      assert_equal('Confirmation sms was sent to your phone. Verification code is 1234.',
+                   flash_message.text)
+    end
+  end
+end