Change test structure to follow closer newer Rails 5 rules

* Create new class called ApplicationIntegrationTest, so we don't have
  to override ActionDispatch::IntegrationTest
* Move UI tests to inherit from ApplicationSystemTestCase
* Existing REST API or EPP tests inherit from
  ApplicationIntegrationTest.
* Move `require 'application_system_test_case'` at the end of
  `test_helper`

I don't particularly agree with the Rails' convention of treating UI
tests as system tests and API tests as integration tests, but I see no
benefit in actively fighting against it.

test/system/admin_area/domains/force_delete_test.rb

@@ -0,0 +1,58 @@
+require 'test_helper'
+
+class AdminAreaDomainForceDeleteTest < ApplicationSystemTestCase
+  include ActionMailer::TestHelper
+
+  setup do
+    sign_in users(:admin)
+    @domain = domains(:shop)
+    ActionMailer::Base.deliveries.clear
+  end
+
+  def test_schedules_domain_force_delete
+    refute @domain.force_delete_scheduled?
+
+    visit edit_admin_domain_url(@domain)
+    click_link_or_button 'Force delete domain'
+    @domain.reload
+
+    assert @domain.force_delete_scheduled?
+    assert_current_path edit_admin_domain_path(@domain)
+    assert_text 'Force delete procedure has been scheduled'
+  end
+
+  def test_notifies_registrar
+    assert_difference '@domain.registrar.messages.size' do
+      visit edit_admin_domain_url(@domain)
+      click_link_or_button 'Force delete domain'
+    end
+  end
+
+  def test_notifies_registrant_and_admin_contacts_by_email_by_default
+    assert_emails 1 do
+      visit edit_admin_domain_url(@domain)
+      click_link_or_button 'Force delete domain'
+    end
+  end
+
+  def test_allows_to_skip_notifying_registrant_and_admin_contacts_by_email
+    assert_no_emails do
+      visit edit_admin_domain_url(@domain)
+      uncheck 'notify_by_email'
+      click_link_or_button 'Force delete domain'
+    end
+  end
+
+  def test_cancels_scheduled_domain_force_delete
+    @domain.update_attribute(:statuses, [DomainStatus::FORCE_DELETE])
+    assert @domain.force_delete_scheduled?
+
+    visit edit_admin_domain_url(@domain)
+    click_link_or_button 'Cancel force delete'
+    @domain.reload
+
+    refute @domain.force_delete_scheduled?
+    assert_current_path edit_admin_domain_path(@domain)
+    assert_text 'Force delete procedure has been cancelled'
+  end
+end