Add a test

test/controllers/signup/completions_controller_test.rb

@@ -29,6 +29,18 @@ class Signup::CompletionsControllerTest < ActionDispatch::IntegrationTest
     assert_response :redirect, "Valid params should redirect"
   end
 
+  test "shows welcome letter after signup" do
+    untenanted do
+      post signup_completion_path, params: {
+        signup: {
+          full_name: @signup.full_name
+        }
+      }
+    end
+
+    assert flash[:welcome_letter]
+  end
+
   test "create with blank name" do
     untenanted do
       post signup_completion_path, params: {