In a debian stretch neither gnu parallel nor moreutils parallel
successfully completes the tests.  Logs are attached. One thing worth
investigating is that gnu parallel inside timeout doesn't really work
right, since it says something like

parallel: SIGTERM received. No new jobs will be started.
parallel: Waiting for these 8 jobs to finish. Send SIGTERM again to stop now.
parallel: /home/bremner/notmuch/test/T357-index-decryption.sh
parallel: /home/bremner/notmuch/test/T330-emacs-subject-to-filename.sh
parallel: /home/bremner/notmuch/test/T310-emacs.sh
parallel: /home/bremner/notmuch/test/T350-crypto.sh
parallel: /home/bremner/notmuch/test/T355-smime.sh
parallel: /home/bremner/notmuch/test/T160-json.sh
parallel: /home/bremner/notmuch/test/T320-emacs-large-search-buffer.sh
parallel: /home/bremner/notmuch/test/T170-sexp.sh

and then doesn't stop. But the real question is why the jobs don't
complete within a 2 minute timeout.

The attached logs are from a stretch chroot running under buster. I also
saw something similar on a stretch system running on real
hardware. Unsurprisingly, the number of tests completed is
non-determinstic, and varies from machine to machine and run to run.