tests: Ensure python tests exit with error on every exception

We ignored assertions happening on callbacks as they only raise
exceptions that does not stop the execution.

So ensure that this is happening in all the tests as synaptics was
doing already

tests/virtual-image.py

@@ -21,6 +21,9 @@ except Exception as e:
 
 FPrint = None
 
+# Exit with error on any exception, included those happening in async callbacks
+sys.excepthook = lambda *args: (traceback.print_exception(*args), sys.exit(1))
+
 def load_image(img):
     png = cairo.ImageSurface.create_from_png(img)