app_helper.rb

module ApplicationHelper
  def weasel(opts={})
    true
  end

  def wrapper
    weasel(1)
  end
end

app_helper_test.rb

require "test_helper"
require "minitest/mock"
require "delegate"

class AppHelper
  include ApplicationHelper
end

class MockAppHelper < AppHelper
  extend Forwardable

  attr_reader :mock

  def_delegator :@mock, :weasel

  def initialize(mock)
    @mock = mock
  end
end

class ApplicationHelperTest < ActionView::TestCase
  setup do
    @helper = AppHelper.new
  end

  test "weasel" do
    assert @helper.weasel
  end

  test "wrapper" do
    mock = Minitest::Mock.new
    mock.expect(:weasel, false, [1])
    mocker = MockAppHelper.new(mock)

    refute mocker.wrapper

    mock.verify
  end
end