doth system test fails with 256 file descriptors

I:doth:checking server quotas for both encrypted and unencrypted HTTP (136)
Traceback (most recent call last):
  File "/Users/marka/git/bind9/bin/tests/system/doth/stress_http_quota.py", line 242, in <module>
    sys.exit(main())
  File "/Users/marka/git/bind9/bin/tests/system/doth/stress_http_quota.py", line 234, in main
    run_test(http_secure=True)
  File "/Users/marka/git/bind9/bin/tests/system/doth/stress_http_quota.py", line 205, in run_test
    subdig.run()
  File "/Users/marka/git/bind9/bin/tests/system/doth/stress_http_quota.py", line 132, in run
    self.sub_process = subprocess.Popen(self.get_command(), shell=True,
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/subprocess.py", line 951, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/subprocess.py", line 1720, in _execute_child
    errpipe_read, errpipe_write = os.pipe()
OSError: [Errno 24] Too many open files
I:doth:failed
I:doth:exit status: 1
I:doth:stopping servers
R:doth:FAIL
E:doth:2021-10-08T12:36:09+1100
FAIL doth (exit status: 1)