Disable available locale checks in Action Test test

Without this change, `store_translations` silently fails when available
locales already initialized.

Ref:
https://travis-ci.org/rails/rails/jobs/497615616#L6846
https://travis-ci.org/rails/rails/jobs/497605027#L6856

actiontext/test/test_helper.rb

@@ -14,6 +14,9 @@
 require "rails/test_unit/reporter"
 Rails::TestUnitReporter.executable = "bin/test"
 
+# Disable available locale checks to allow to add locale after initialized.
+I18n.enforce_available_locales = false
+
 # Load fixtures from the engine
 if ActiveSupport::TestCase.respond_to?(:fixture_path=)
   ActiveSupport::TestCase.fixture_path = File.expand_path("fixtures", __dir__)