tests: Fix another clang compiler warning

Compiling with clang 21.1.2 leads to following warning:

```
../glib/tests/cxx.cpp:566:1: warning: non-void function does not return a value in all control paths [-Wreturn-type]
  566 | }
```

Add the theoretically missing return statement, even though the if-block
should always be reached. Since no other compiler warned about the
unreachable statement with this change, this should be okay.

glib/tests/cxx.cpp

@@ -563,6 +563,7 @@ g_likely_test_expr (void)
     {
       return TRUE;
     }
+  return FALSE;
 }
 
 static void