tests: add tests for when stdout or stderr is connected to `os.devnull`
The original motivation was that creating PTYs on Windows is not possible, but
`NUL` is recognized as a TTY, so we can have at least some test coverage for
the TTY case. I think it doesn’t hurt to run the test cases on all systems.
$ LANG=nonexistent LC_ALL=nonexistent LANGUAGE=nonexistent hg version -q
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