Add tests of autoprocessing

[rpanderson]

Tests autoscrub autoprocess command (by calling the CLI) on example_screencast.mp4, a file previously used for benchmarking and troubleshooting.

• Check for successful completion of command.
• Compare filtergraph to known valid ouput.
• Check duration of processed file.
• Check loudness statistics of processed file.

[rpanderson]

Using Github Actions to run pytest on ubuntu-latest (actually 18.04.4), which passed.

Notes:

• The latest FFmpeg on Ubuntu 18.04 is 3.4.4. This results in slightly different processing than FFmpeg 4.x, e.g. duration is 50.19s instead of 48.57s, some loudness values differ by 0.1 dB, and the filtergraph is therefore quite different.
• I install 4.x using ppa:jonathonf/ffmpeg-4, which currently gets 4.2.2. There are others, e.g. ppa:savoury1/ffmpeg4.
• This might have to revised when ubuntu-latest installs Ubuntu 20.04, or use snappy.
• The line endings for the temporary filtergraph written by autoprocess differ on Windows (CRLF) and Linux (LF), so the strict filecmp.cmp call will currently only work for one of these (Linux).

TODO

• Test on different Python versions (2.7, 3.4+) using GitHub actions matrix strategy. Successful check here.
• Make filtergraph comparison independent of line endings.
• Test on different operating systems. with OS-dependent dependency installation.

[rpanderson]

Per #2 in which processing continued indefinitely, a timeout is essential here, akin to subprocess.Popen.communicate. @philipstarkey, is this something that could/should be added to autoscrub._agnostic_communicate or should I be using something else. here?

[rpanderson]

For now I've set a timeout on the test runner, using pytest-timeout.

[rpanderson]

Not sure why, but the test is stalling on Windows at this point (both locally and on Python 2.7 and 3.8).