add minimal testing for question

spec/models/form/lettings/questions/net_income_known_spec.rb

@@ -0,0 +1,17 @@
+require "rails_helper"
+
+RSpec.describe Form::Lettings::Questions::NetIncomeKnown do
+  subject(:question) { described_class.new(nil, {}, nil) }
+
+  describe "#id" do
+    it "is net_income_known" do
+      expect(question.id).to eql("net_income_known")
+    end
+  end
+
+  describe "#type" do
+    it "is radio" do
+      expect(question.type).to eql("radio")
+    end
+  end
+end