tests(PythonQtTestCleanup): Fix unknown attribute in testCallQtMethodInDel

This change ensures that the `QTimer` object is properly deleted to avoid the
AttributeError when the `__del__` method is called.

Fixes the following error:

```
Exception ignored in: <function TimerWrapper.__del__ at 0x510000073760>
Traceback (most recent call last):
  File "<string>", line 6, in __del__
AttributeError: 'QTimer' object has no attribute 'setSingleShot'
```

Author: jcfr

tests/PythonQtTestCleanup.cpp

@@ -54,6 +54,7 @@ void PythonQtTestCleanup::testCallQtMethodInDel()
     "  def __del__(self):\n" \
     "    self.timer.setSingleShot(True)\n" \
     "x = TimerWrapper()\n" \
+    "del x\n" \
     "obj.setPassed()\n"
     ));
 }