Allow :no_op to be specified as an input value

[acant]

The idea being that for some scenarios we do not all the variables to be used. (e.g., a method which has several guard clauses which each check one or more inputs).

Given a method like this:

def method_under_set(input1, input2, input3)
  return unless is_valid(input1)
  return unless is_valid(input2)
  return unless is_valid(input3)

  input1 + input2 + input3
end

I might have a tabular spec like this:

inputs :input1, :input2, :input3
it_with :value1, nil, nil, nil
it_with :value1, :value2, nil, nil
it_with :value1, :value2, :value3, :result

But this does not clearly communicate that the inputs should not actually be used in the first 2 examples. It makes it hard to see which examples expect an actual nil.

So I would like to have:

inputs :input1, :input2, :input3
it_with :value1, :no_op, :no_op, nil
it_with :value1, :value2, :no_op, nil
it_with :value1, :value2, :value3, :result

The variable with the :no_op value could make use of the memoizing/let blocks in order to raise an assertion if the variable is called during the scenario. But that might be unnecessarily complex.