licenses test: Add a smoke test for additionalLicenses

Skipped for now, because of #1540.

Author: rajveermalviya

test/licenses_test.dart

@@ -0,0 +1,14 @@
+import 'package:checks/checks.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:zulip/licenses.dart';
+
+import 'fake_async.dart';
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+
+  test('smoke: ensure all additional licenses load', () => awaitFakeAsync((async) async {
+    await check(additionalLicenses().toList())
+      .completes((it) => it.isNotEmpty());
+  }), skip: true); // TODO(#1540)
+}