Follow up on newly-marked test xfails #330
Description
In #329, we added two new xfail as a part of getting the tests running in CI. These tests appear to have broken from some drift in brozzler's internal APIs since the tests were written.
test_units.test_proxy_down attempts to call brozzler.ydl.do_youtube_dl with three arguments; we added a mandatory fourth argument in baa33e3.
test_units.test_needs_browsing attempts to call brozzler.ydl.YoutubeDLSpy, a class that was removed in 487a700.