test-resultstree: fixed -Wuseless-cast GCC warnings in moc_*.cpp

example:
```
/home/runner/work/cppcheck/cppcheck/cmake.output.tinyxml2/gui/test/resultstree/__/__/moc_threadhandler.cpp: In static member function ‘static void ThreadHandler::qt_static_metacall(QObject*, QMetaObject::Call, int, void**)’:
/home/runner/work/cppcheck/cppcheck/cmake.output.tinyxml2/gui/test/resultstree/__/__/moc_threadhandler.cpp:100:51: error: useless cast to type ‘using _t = void (class ThreadHandler::*)()’ {aka ‘void (class ThreadHandler::*)()’} [-Werror=useless-cast]
  100 |             if (*reinterpret_cast<_t *>(_a[1]) == static_cast<_t>(&ThreadHandler::done)) {
      |                                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

Author: firewave

gui/test/resultstree/CMakeLists.txt

@@ -45,6 +45,9 @@ if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
     endif()
     # caused by mocks
     target_compile_options_safe(test-resultstree -Wno-missing-noreturn)
+elseif (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+    # caused by mocks
+    target_compile_options_safe(test-resultstree -Wno-useless-cast)
 endif()
 
 if (REGISTER_GUI_TESTS)