Show accepted signup delivery without a second send step (#62)

frontend/packages/sxchnge_auth/test/auth_test.dart

@@ -142,11 +142,11 @@ void main() {
           .isSimulated,
       isTrue,
     );
-    expect(
-      (await const DummySignupService(delay: Duration.zero).signup(signup))
-          .isSimulated,
-      isTrue,
-    );
+    final accepted = await const DummySignupService(delay: Duration.zero)
+        .signup(signup);
+    expect(accepted.isSimulated, isTrue);
+    expect(accepted.delivery?.email, 'a@example.com');
+    expect(accepted.delivery?.isSimulated, isTrue);
     await expectLater(
       const DummyLoginService(
         delay: Duration.zero,