If I move the close to before the support.waitprocess() call I get:

aixtools@x064:[/home/aixtools/py39-3.9]./python -m test test_builtin
0:00:00 Run tests sequentially
0:00:00 [1/1] test_builtin

I am child - this is my PID:22544444
I am parent:21954666 with child:22544444fd:6 input:b'quux\r'
I am parent:21954666 with lines:[]
I am child - exiting PID:22544444
I am parent:21954666 with lines:['stdin.isatty(): True']
I am parent:21954666 with lines:['stdin.isatty(): True', "captured: 'prompt'"]
I am parent:21954666 starting wait_process(22544444, exitcode=0)
I am parent:21954666 with child:22544446fd:6 input:b'quux\r\n'
I am parent:21954666 with lines:[]
I am child - this is my PID:22544446
I am child - exiting PID:22544446
I am parent:21954666 with lines:['tty = True']
I am parent:21954666 with lines:['tty = True', "'quux'"]
I am parent:21954666 starting wait_process(22544446, exitcode=0)
I am parent:21954666 with child:22544448fd:6 input:b'quux\xe9\r\n'
I am parent:21954666 with lines:[]
I am child - this is my PID:22544448
I am child - exiting PID:22544448
I am parent:21954666 with lines:['tty = True']
I am parent:21954666 with lines:['tty = True', "'quux\\udce9'"]
I am parent:21954666 starting wait_process(22544448, exitcode=0)
I am parent:21954666 with child:22544450fd:6 input:b'quux\xe9\r\n'
I am parent:21954666 with lines:[]
I am child - this is my PID:22544450
I am child - exiting PID:22544450
I am parent:21954666 with lines:['tty = True']
I am parent:21954666 with lines:['tty = True', "'quux\\udce9'"]
I am parent:21954666 starting wait_process(22544450, exitcode=0)
== Tests result: SUCCESS ==

1 test OK.