add test for joint purchase question

spec/models/form/sales/pages/joint_purchase_spec.rb

@@ -0,0 +1,29 @@
+require "rails_helper"
+
+RSpec.describe Form::Sales::Pages::JointPurchase, type: :model do
+  subject(:page) { described_class.new(page_id, page_definition, subsection) }
+
+  let(:page_id) { nil }
+  let(:page_definition) { nil }
+  let(:subsection) { instance_double(Form::Subsection) }
+
+  it "has correct subsection" do
+    expect(page.subsection).to eq(subsection)
+  end
+
+  it "has correct questions" do
+    expect(page.questions.map(&:id)).to eq(%w[type])
+  end
+
+  it "has the correct id" do
+    expect(page.id).to eq("joint_purchase")
+  end
+
+  it "has the correct header" do
+    expect(page.header).to eq("")
+  end
+
+  it "has the correct description" do
+    expect(page.description).to eq("")
+  end
+end