tst_moc: fix returning-reference-to-local warning

Although the code is never executed compilers still throw a warning
because it's compiled.

Amends 12b8283f89

Pick-to: 5.15 6.1
Change-Id: Ib790d4bcb33c4b9f2a55a784b852275b59debde9
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

tests/auto/tools/moc/cxx11-trailing-return.h

@@ -43,12 +43,12 @@ public slots:
 
     inline auto constRefReturn() -> const CXX11TrailingReturn &
     {
-        return {};
+        return *this;
     }
 
     inline auto constConstRefReturn() const -> const CXX11TrailingReturn &
     {
-        return {};
+        return *this;
     }
 
 signals: