I'm not sure that a functional test is required here to validate manually that buffered are flushed properly. Maybe testing the following attributes are enough?

>>> sys.stdout.write_through, sys.stdout.write_through
(False, False)

What do you think @pitrou?

My worry is that functional tests are more likely to fail and so give me more to work to maintain the CI :-)

I added non-functional tests and removed the functional tests.

I suggest to rewrite the test to run Python a single time rather than 6 times, to make the test faster:

    def test_non_interactive_output_buffering(self):
        code = textwrap.dedent("""
            import sys
            out = sys.stdout
            print(out.isatty(), out.write_through, out.line_buffering)
            err = sys.stderr
            print(err.isatty(), err.write_through, err.line_buffering)
        """)
        args = [sys.executable, '-c', code]
        proc = subprocess.run(args, stdout=subprocess.PIPE,
                              text=True, check=True)
        self.assertEqual(proc.stdout,
                         'False False False\n'
                         'True False True\n')

Good point! I used your code, but added stderr=subprocess.PIPE to ensure that sys.stderr is not a TTY (without this both the old and the new code would pass the test).