We've recently added warning log messages to the FFMPEGMuxer's resolver when FFmpeg could not be found, so that the user can be informed that muxing is not supported. However, there are still a couple of issues left which I think should be addressed:

1. The chosen/resolved FFmpeg executable does not get validated/verified:
   This is a problem when the user has set an invalid FFmpeg executable, or when it is broken and doesn't work. In case of invalid executables, the output and stream muxing are pretty much undefined, and broken executables lead to empty stream outputs, and unless verbose logging is enabled, no error messages get printed, making the user believe there's an issue with the input stream.
2. The FFmpeg version does not get logged:
   Logging the FFmpeg version makes debugging issues easier. The recent issue which comes to mind is:
   stream.ffmpegmux: stream with independent segments (audio+video) cannot be remuxed #4825
3. Certain streams might require a certain FFmpeg version in order to correctly mux streams:
   This PR doesn't add any version checks, but this could be added in the future if it's needed, based on this work.
   hls-multi from arte.tv has seconds of hanging video #4520 (comment)

Opening the PR as a draft, as it's unfinished work.

The first commit implements a generic ProcessOutput utility class for asynchronously spawning a subprocess and reading its stdout/stderr streams.

This is necessary, because using APIs like subprocess.run(capture_output=True) or asyncio.create_subprocess_exec().communicate() read the entire stdout/stderr streams into memory until the process terminates on its own. This is bad when executing unknown binaries. And reading streams of a spawned subprocess in separate threads as well as dealing with timeouts is unnecessarily difficult to implement in regards to making everything thread-safe (especially when writing tests, just see the messy HLS stream tests for example).

Asynchronously iterating output streams and using asyncio.sleep() makes this fairly easy and straightforward to implement, but testing stuff unfortunately requires adding two more dev-requirements:

1. pytest-asyncio - already installed as a transitive dependency, so no changes here
2. mock - compatibility backport of the unittest.mock stdlib implementation for py37 (AsyncMock is missing)
   mock was already removed from the dev-requirements in d04767f, and the only gotchas were the args/kwargs properties in Mock.call_args which are missing on py37, but that doesn't justify adding compatibility imports everywhere

Another issue I ran into while writing tests was freezegun, which currently affects the internals of asyncio in its most recent version, and there's an open PR on their repo which will change that. I managed to get the tests working (after lots of issues with asyncio.sleep()), but I'm not sure how the next freezegun release(s) will affect the tests. We'll see...

The second commit adds FFmpeg executable validation and version logging. The validation can be turned off by setting --ffmpeg-no-validation.

Still need to update and fix tests.

I've included some links in the code to the FFmpeg version string definitions, which have been stable for 12 years now.