Rails 3.2 - test for application_helper.rb fails when trying to access application_controller method

I am trying to test my application_helper methods, and some of them are
calling ApplicationController methods

class ApplicationHelperTest < Test::Unit::TestCase

test ‘test_copyrights’ do
assert_equal @subdomain[:name], copyright
end

class ApplicationController < ActionController::Base
helper_method :current_subdomain
def current_subdomain

end

end

but the test is failing :
NameError: undefined local variable or method `current_subdomain’

how can I test it ? ( where… in unit/functional/integration test ?
and
how to access the ApplicationController methods ?)

thanks for feedback