STM Weak test parallel fail to trigger on s390x

[jmid]

On #403 we observed that STM Weak test parallel failed to trigger an error on s390x in 2000 attempts:
https://ocaml-multicoretests.ci.dev:8100/job/2023-11-01/092539-ci-ocluster-build-7fa25a

random seed: 311406750
generated error fail pass / total     time test name

[ ]    0    0    0    0 / 1000     0.0s STM Weak test sequential
[ ]    0    0    0    0 / 1000     0.0s STM Weak test sequential (generating)
[✓] 1000    0    0 1000 / 1000     0.2s STM Weak test sequential
================================================================================
success (ran 1 tests)
generated error fail pass / total     time test name

[ ]    0    0    0    0 / 2000     0.0s STM Weak test parallel
[ ]  721    0    0  721 / 2000    59.8s STM Weak test parallel
[ ] 1761    0    0 1761 / 2000   119.8s STM Weak test parallel
[✗] 2000    0    0 2000 / 2000   131.1s STM Weak test parallel

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

Test STM Weak test parallel failed:

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