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/integration/epp/domain/create/nameservers_test.rb

@@ -1,6 +1,6 @@
 require 'test_helper'
 
-class EppDomainCreateNameserversTest < ActionDispatch::IntegrationTest
+class EppDomainCreateNameserversTest < ApplicationIntegrationTest
   # Glue record requirement
   def test_nameserver_ip_address_is_required_if_hostname_is_under_the_same_domain
     request_xml = <<-XML

test/integration/epp/domain/create/transfer_code_test.rb

@@ -1,6 +1,6 @@
 require 'test_helper'
 
-class EppDomainCreateTransferCodeTest < ActionDispatch::IntegrationTest
+class EppDomainCreateTransferCodeTest < ApplicationIntegrationTest
   setup do
     travel_to Time.zone.parse('2010-07-05')
   end

test/integration/epp/domain/domain_delete_test.rb

@@ -1,6 +1,6 @@
 require 'test_helper'
 
-class EppDomainDeleteTest < ActionDispatch::IntegrationTest
+class EppDomainDeleteTest < ApplicationIntegrationTest
   def test_bypasses_domain_and_registrant_and_contacts_validation
     request_xml = <<-XML
       <?xml version="1.0" encoding="UTF-8" standalone="no"?>

test/integration/epp/domain/domain_renew_test.rb

@@ -1,6 +1,6 @@
 require 'test_helper'
 
-class EppDomainRenewTest < ActionDispatch::IntegrationTest
+class EppDomainRenewTest < ApplicationIntegrationTest
   self.use_transactional_fixtures = false
 
   setup do

test/integration/epp/domain/domain_update_test.rb

@@ -1,6 +1,6 @@
 require 'test_helper'
 
-class EppDomainUpdateTest < ActionDispatch::IntegrationTest
+class EppDomainUpdateTest < ApplicationIntegrationTest
   def test_update_domain
     request_xml = <<-XML
       <?xml version="1.0" encoding="UTF-8" standalone="no"?>

test/integration/epp/domain/transfer/base_test.rb

@@ -1,6 +1,6 @@
 require 'test_helper'
 
-class EppDomainTransferBaseTest < ActionDispatch::IntegrationTest
+class EppDomainTransferBaseTest < ApplicationIntegrationTest
   def test_non_existent_domain
     request_xml = <<-XML
       <?xml version="1.0" encoding="UTF-8" standalone="no"?>

test/integration/epp/domain/transfer/query_test.rb

@@ -1,6 +1,6 @@
 require 'test_helper'
 
-class EppDomainTransferQueryTest < ActionDispatch::IntegrationTest
+class EppDomainTransferQueryTest < ApplicationIntegrationTest
   def test_returns_domain_transfer_details
     post '/epp/command/transfer', { frame: request_xml }, { 'HTTP_COOKIE' => 'session=api_bestnames' }
     xml_doc = Nokogiri::XML(response.body)

test/integration/epp/domain/transfer/request_test.rb

@@ -1,6 +1,6 @@
 require 'test_helper'
 
-class EppDomainTransferRequestTest < ActionDispatch::IntegrationTest
+class EppDomainTransferRequestTest < ApplicationIntegrationTest
   setup do
     @domain = domains(:shop)
     @new_registrar = registrars(:goodnames)

test/integration/epp/login/credentials_test.rb

@@ -1,6 +1,6 @@
 require 'test_helper'
 
-class EppLoginCredentialsTest < ActionDispatch::IntegrationTest
+class EppLoginCredentialsTest < ApplicationIntegrationTest
   def test_correct_credentials
     request_xml = <<-XML
       <?xml version="1.0" encoding="UTF-8" standalone="no"?>

test/integration/epp/login/session_limit_test.rb

@@ -1,6 +1,6 @@
 require 'test_helper'
 
-class EppLoginSessionLimitTest < ActionDispatch::IntegrationTest
+class EppLoginSessionLimitTest < ApplicationIntegrationTest
   setup do
     travel_to Time.zone.parse('2010-07-05')
     EppSession.delete_all

test/integration/epp/logout_test.rb

@@ -1,6 +1,6 @@
 require 'test_helper'
 
-class EppLogoutTest < ActionDispatch::IntegrationTest
+class EppLogoutTest < ApplicationIntegrationTest
   def test_success_response
     post '/epp/session/logout', { frame: request_xml }, { 'HTTP_COOKIE' => 'session=api_bestnames' }
     assert Nokogiri::XML(response.body).at_css('result[code="1500"]')

test/integration/epp/poll_test.rb

@@ -1,6 +1,6 @@
 require 'test_helper'
 
-class EppPollTest < ActionDispatch::IntegrationTest
+class EppPollTest < ApplicationIntegrationTest
   def test_messages
     post '/epp/command/poll', { frame: request_xml }, { 'HTTP_COOKIE' => 'session=api_bestnames' }
     assert_equal '1301', Nokogiri::XML(response.body).at_css('result')[:code]