FreeBSD failure to trigger 'STM Sys test parallel'

[jmid]

In #409 a FreeBSD CI run on 5.0.0 failed to trigger a counterexample in STM Sys test parallel within 1000 attempts:
https://ocaml-multicoretests.ci.dev:8100/job/2023-11-09/084832-ci-ocluster-build-542efa

random seed: 4002063
generated error fail pass / total     time test name

[ ]    0    0    0    0 / 1000     0.0s STM Sys test sequential
[ ]    0    0    0    0 / 1000     0.0s STM Sys test sequential (generating)
[✓] 1000    0    0 1000 / 1000     4.8s STM Sys test sequential

[ ]    0    0    0    0 / 1000     0.0s STM Sys test parallel
[ ]  512    0    0  512 / 1000    55.3s STM Sys test parallel
[✗] 1000    0    0 1000 / 1000   115.0s STM Sys test parallel

--- Failure --------------------------------------------------------------------

Test STM Sys test parallel failed:

Negative test STM Sys test parallel succeeded but was expected to fail
================================================================================
failure (1 tests failed, 0 tests errored, ran 2 tests)
File "src/sys/dune", line 4, characters 7-16:
4 |  (name stm_tests)
           ^^^^^^^^^
(cd _build/default/src/sys && ./stm_tests.exe --verbose)
Command exited with code 1.