Having some trouble testing module instance method behavior.

example_stub_problem.rb

module Foo
  def bar
    "#{to_s} whizbang"
  end
end

describe Foo do
  let(:klass) do
    Class.new do
      include Foo
    end
  end
  
  subject{ klass.new }
  
  context "#bar" do
    it "works" do
      expect(subject).to receive(:to_s).and_return "test"
      expect(subject.bar).to eq "test whizbang"
    end
  end
end