tests: make smoke-tests Qt6-safe (use QStringLiteral/QLatin1)

Lts/DDS/DdsUnitTestsPlaceholder/ddsDataTopicTests_smoke.cpp

@@ -9,8 +9,8 @@ private slots:
     QVERIFY(QCoreApplication::instance() == nullptr || true); // harmless check
   }
   void test_qstring_format() {
-    QString fmt = QStringLiteral("%1-%2").arg(42).arg("abc");
-    QCOMPARE(fmt, QString("42-abc"));
+    QString fmt = QStringLiteral("%1-%2").arg(42).arg(QStringLiteral("abc"));
+    QCOMPARE(fmt, QStringLiteral("42-abc"));
   }
 };