ChefSpec Unit Testing: Expect custom resource to call helper library method

Chef InSpec
1

Given the following setup

Helper Library

Class Foo
   def self.bar(x)
      puts x
   end
end

Custom resource

provides :foo 
default_action :quux

property :bar, String

load_current_value do
  bar 'not blah'
end

action :quux do
  converge_if_changed :bar do
    converge_by 'calling Foo.bar' do
       Foo.bar(new_resource.bar)
    end
  end
end

How do I test that the helper method is called

describe "resource calls helper method" do
  before do
     # *** I know that the helper method can be stubbed like this ***
     allow(Foo).to receive(:bar).and_return 'foo'
  end
  recipe do
    foo 'blah' do
      bar 'blah'
    end
  end

  # *** I know I can test Chef built-in resources like this ***
  it { is_expected.to run_execute('blah') }

  ######################################################
  # HOW DO I TEST THAT THE HELPER METHOD WAS CALLED??? 
  # The following does not work
  #######################################################
  it { expect(Foo).to receive(:bar).with('blah') 
end