test: test.py leaks pseudo-tty master fd to child

[bnoordhuis]

Lest I forget: the master fd created here:

node/tools/test.py

Line 723 in 20d099a

(out_master, fd_out) = pty.openpty()

Is leaked to the spawned child here:

node/tools/test.py

Lines 761 to 772 in 20d099a

	(process, exit_code, timed_out, output) = RunProcess(
	context,
	timeout,
	args = args,
	stdin = fd_in,
	stdout = fd_out,
	stderr = fd_err,
	env = env_copy,
	faketty = faketty,
	pty_out = pty_out,
	preexec_fn = preexec_fn
	)

Fix: the preexec_fn should os.close(pty_out).

[bnoordhuis]

#28600 should fix the pty hangs on AIX. It came a long way...