RSpec/InstanceVariable should ignore dynamic classes

[deivid-rodriguez]

We have some specs we we define dynamic classes inside specs like this

describe Decidim::Exporters::CSV do
  let(:serializer) do
    Class.new do
      def initialize(resource)
        @resource = resource
      end

      def serialize
        {
          id: @resource.id,
          serialized_name: @resource.name,
          other_ids: @resource.ids
        }
      end
    end
  end

  # my specs ...
end

RSpec/InstanceVariable should not fire in this case.