tests: check that procutil.std{out,err}.write() returns correct result On Windows, we currently don’t fully test the case when the stream is connected to a TTY, but we test the child process side by connecting them to NUL, which is recognized as a TTY by Python. To make the large write test a bit more useful besides checking that it doesn’t crash, we can check that the write() method returns the correct result.

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.