[3.12] gh-110673: test_pty raises on short write (GH-110677) (#110742)

gh-110673: test_pty raises on short write (GH-110677)

Add write_all() helper function to test_pty to raise an exception on
short write: if os.writes() does not write all bytes. It should not
happen for a PTY.
(cherry picked from commit b4e8049766a46a9e6548b18d7e9a0c9f573cd122)

Co-authored-by: Victor Stinner <vstinner@python.org>