I've added a new test which checks the formatter's initialization and the correct metadata variable defintions. This still doesn't test every change in the main CLI module, but I don't care, because adding more and more tests with 100s of mocks for this aged module doesn't make sense when the design isn't good to begin with. As said, a complete rewrite is needed.
https://app.codecov.io/gh/streamlink/streamlink/compare/3962/diff

Also just noticed that my test changes fix a mistake which was present since several years where it always created an HTTPServer instance when running file output tests:

• https://app.codecov.io/gh/streamlink/streamlink/compare/3962/changes#D1L104

• streamlink/tests/test_cli_main.py

  Lines 251 to 260 in 05bdf99

  	streamlink_cli.main.args = args = Mock()
  	args.output = None
  	args.stdout = None
  	args.record = tmpfile.name
  	args.record_and_pipe = None
  	args.title = None
  	args.player = "mpv"
  	args.player_args = ""
  	args.player_fifo = None
  	self.assertIsInstance(create_output(FakePlugin), PlayerOutput)

• streamlink/src/streamlink_cli/main.py

  Lines 101 to 107 in 05bdf99

  	if args.player_fifo:
  	try:
  	namedpipe = NamedPipe()
  	except OSError as err:
  	console.exit("Failed to create pipe: {0}", err)
  	elif args.player_http:
  	http = create_http_server()

A future PR could add more vars to the formatter, with optional custom format specifiers (if that's possible):

• time: {date}, {time}, etc.
• stream container: {filetype}, {ending}, etc.