appiumtests: stop GLib main loop before player quit

Fixes an exception in threading:

```
Exception ignored in: <module 'threading' from '/usr/lib64/python3.11/threading.py'>
Traceback (most recent call last):
  File "/usr/lib64/python3.11/threading.py", line 1590, in _shutdown
    lock.acquire()
  File "/builds/plasma/plasma-workspace/appiumtests/utils/mediaplayer.py", line 470, in __on_terminate
    sys.exit(0)
```