Add failing test for undefined method

spec/oscal/assessment_plan_spec.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+require_relative "../spec_helper"
+
+RSpec.describe Oscal::AssessmentPlan::AssessmentPlan do
+  let(:fields) do
+    {
+      uuid: "4d56d1ab-d91e-4f7a-9055-1883c4e580b8",
+      metadata: {},
+      import_ssp: { href: "./ssp.json" },
+      reviewed_controls: { control_selections: [] },
+    }
+  end
+  subject { described_class.new(fields) }
+
+  describe "#to_json" do
+    it "generates a json representation of the assessment plan" do
+      expect(subject.to_json).to be_kind_of(String)
+    end
+  end
+end

spec/oscal/catalog_spec.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require_relative "../spec_helper"
+
 RSpec.describe Oscal::Catalog do
   let(:subject) do
     Oscal::Catalog.load_from_yaml(