gistfile1.rb

...
let(:object) { Testing::Test.new }
let(:run_command) { stub(:stdout => 'Some string') }
let(:shellout) { stub(:run_command => run_command) }

it 'shells out' do
  Mixlib::ShellOut.stub(:new).and_return(shellout)
  expect(object.shellout).to eq('Some string')
end
...