Check for tests mkv files by filenames, not amount

[ptrcnull]

Currently we're checking how many files exist in the mkv directory, and redownloading the entire 180MB archive when the file count doesn't match:

subliminal/tests/conftest.py

Line 443 in 929424c

if not os.path.exists(data_path) or len(os.listdir(data_path)) != 8:

This is a bit silly, since in build processes where you don't have access to internet you'd just extract the entire zip file, which contains a bit more than 8 files in total; instead, we should check if the files we want exist in the directory, somewhat like enzyme is doing currently:
https://github.com/Diaoul/enzyme/blob/e3086c3222cf732b63d35c31165ac20304dd2590/tests/test_mkv.py#L20