Missed add: deprecated erb_variable test

actionpack/test/template/deprecated_erb_variable_test.rb

@@ -0,0 +1,9 @@
+require 'abstract_unit'
+
+class DeprecatedErbVariableTest < ActionView::TestCase
+  def test_setting_erb_variable_warns
+    assert_deprecated 'erb_variable' do
+      ActionView::Base.erb_variable = '_erbout'
+    end
+  end
+end