Add idempotence rules to the DSL

[mavnn]

Experimental idea: this PR adds "idempotence rules" to edge tests, allowing you to override values that might change from run to run of the edge tests.

pin_value pins a value of a specific field in the JSON:

controller ApplicationController do
  action :home do
    edge "write elm file" do
      perform_get
      pin_value(["key1", "key2"], 10)
      produce_elm_file('Home')
    end
  end
end

will error if the JSON created doesn't contain an item at json_data["key1"]["key2"]. If it does contain an item, it will override it with the pinned value supplied (10).

The more general add_idempotence_rule just allows you to supply a block which can carry out arbitrary transformations on the generated JSON hash:

controller ApplicationController do
  action :home do
    edge "write elm file" do
      perform_get
      add_idempotence_rule do |json_data|
        json_data["key1"]["key2"] = 10
      end
      produce_elm_file('Home')
    end
  end
end