bpo-40140: test_builtin.PtyTests registers SIGHUP handler (GH-19314)

bpo-40140: test_builtin.PtyTests registers SIGHUP handler (GH-19314)

test_builtin.PtyTests now registers an handler for SIGHUP signal.
Closing the PTY file descriptor can emit a SIGHUP signal: just ignore
it.

run_child() now also closes the PTY file descriptor before waiting
for the process completition, otherwise the test hangs on AIX.