thumbnailer: Fix cancellation behavior & associated test

This now emits the completion callback upon a thumbnail request cancellation. It matches the documented libvlc behavior (which was incorrect since the libvlc callback was not invoked after a cancellation)

The code now behaves as the test expected, which should make it pass deterministically, while it was sporadically succeeding due to an uninitialized value. This use of uninitialized value is also fixed by this MR