Extract shared example for cacheable response in specs (#25388)

spec/support/examples/cache.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+shared_examples 'cacheable response' do |expects_vary: false|
+  it 'does not set cookies' do
+    expect(response.cookies).to be_empty
+    expect(response.headers['Set-Cookies']).to be_nil
+  end
+
+  it 'does not set sessions' do
+    expect(session).to be_empty
+  end
+
+  if expects_vary
+    it 'returns Vary header' do
+      expect(response.headers['Vary']).to include(expects_vary)
+    end
+  end
+
+  it 'returns public Cache-Control header' do
+    expect(response.headers['Cache-Control']).to include('public')
+  end
+end