[test_download] Print test names in case of network errors
[test_download] Add comments for nose parameters
[test_download] Modify outtmpl to prevent info JSON filename conflicts
Thanks @jaimeMF for the idea.
[travis] Only download tests should be run in parallel
In pycodestyle 2.1.0, E305 was introduced, which requires two blank
lines after top level declarations, too.
See https://github.com/PyCQA/pycodestyle/issues/400
See also #10689; thanks @stepshal for first mentioning this issue and
initial patches
From now on, test_download will run these tests. That means we benefit not only from the networking setup in there, but also from the other tests (for example test_all_urls to find problems with _VALID_URLs).
This moves playlist tests where they belong, i.e. to the extractors themselves.
Additionally, all our network interaction configuration for tests in test_download now applies to playlist tests as well.
If the result is not testable (for example, because a description changes often), either pass in a type or a regular expression (a string starting with 're:')