[Tests] Keep the icon render test QApplication alive until exit

Creating it as a function-local static tears it down before Qt's library statics
at exit, and libQt6Network then runs QObject destructors against freed memory —
a SEGV the sanitizer leg reports after the tests have already passed. Leak the
application instead, the usual pattern for a test binary.

tests/theme_migration/theme_icon_render_test.cpp

@@ -28,7 +28,10 @@ void ensureGuiApplication()
     static int argc = 1;
     static char name[] = "theme_icon_test";
     static char *argv[] = {name, nullptr};
-    static QApplication app(argc, argv);
+    // Leaked on purpose: a function-local static QApplication is destroyed before Qt's
+    // own library statics at exit, and the network module's destructors then run
+    // against a torn-down QObject layer (SEGV under AddressSanitizer).
+    static QApplication *app = new QApplication(argc, argv);
     Q_UNUSED(app);
 }