[YouTube] Check if videos with multiple uploaders are handled correctly

[litetex]

I just discovered that it's now possible that there are multiple uploaders for a single video:
#1320 (comment)

https://www.youtube.com/watch?v=Nxk6aRHi664

We should check if the extractor can process such videos correctly.
Maybe we should also add a test (after #1332 was merged)