Cop idea: Prevent using expect in let.

[marocchino]

I've noticed that some people use let as an escape from the before hook to avoid RSpec/ExpectInHook.

so..

  let(:expect_let) do
    expect(Module).to receive(:call)
  end

  before do
    expect_let
  end

will not trigger RSpec/ExpectInHook.

There is no good reason to put expect inside let in the first place, so I think it would be better to disallow it.