Reliability of parallel Lin Out_channel tests

[jmid]

Currently we are disabling Out_channel tests on macOS because their output proved too unreliable:

multicoretests/src/io/lin_tests_dsl_domain.ml

Lines 12 to 14 in 6a549cd

	if Sys.getenv_opt "OCAML_SYSTEM" = Some "macosx"
	then (
	Printf.printf "Lin DSL Out_channel test with Domain disabled under macOS\n\n%!";

I expect that this will also benefit the test signal coming from the new shiny arm64 macOS runners from Multicoretests-CI.

However the test unfortunately doesn't reliably fail on other targets.

Here's a Windows bytecode run with 5.1.0~alpha2 from 2 days ago that failed to trigger:
https://github.com/ocaml-multicore/multicoretests/actions/runs/5173787089/jobs/9319393810

random seed: 217324792
generated error fail pass / total     time test name

[ ]    0    0    0    0 / 1000     0.0s Lin DSL In_channel test with Domain
[ ]    0    0    0    0 / 1000     0.0s Lin DSL In_channel test with Domain (generating)
[✓]   13    0    1   12 / 1000    15.0s Lin DSL In_channel test with Domain

[ ]    0    0    0    0 / 1000     0.0s Lin DSL Out_channel test with Domain
[ ]    3    0    0    3 / 1000   306.4s Lin DSL Out_channel test with Domain
[ ]   20    0    0   20 / 1000   372.0s Lin DSL Out_channel test with Domain
[ ]   43    0    0   43 / 1000   434.1s Lin DSL Out_channel test with Domain
[ ]   76    0    0   76 / 1000   495.0s Lin DSL Out_channel test with Domain
[ ]  108    0    0  108 / 1000   558.8s Lin DSL Out_channel test with Domain
[ ]  129    0    0  129 / 1000   619.0s Lin DSL Out_channel test with Domain
[ ]  168    0    0  168 / 1000   684.1s Lin DSL Out_channel test with Domain
[ ]  217    0    0  217 / 1000   744.5s Lin DSL Out_channel test with Domain
[ ]  262    0    0  262 / 1000   805.0s Lin DSL Out_channel test with Domain
[ ]  311    0    0  311 / 1000   867.0s Lin DSL Out_channel test with Domain
[ ]  387    0    0  387 / 1000   928.1s Lin DSL Out_channel test with Domain
[ ]  444    0    0  444 / 1000   990.5s Lin DSL Out_channel test with Domain
[ ]  525    0    0  525 / 1000  1051.7s Lin DSL Out_channel test with Domain
[ ]  608    0    0  608 / 1000  1113.5s Lin DSL Out_channel test with Domain
[ ]  698    0    0  698 / 1000  1177.1s Lin DSL Out_channel test with Domain
[ ]  794    0    0  794 / 1000  1237.4s Lin DSL Out_channel test with Domain
[ ]  839    0    0  839 / 1000  1305.8s Lin DSL Out_channel test with Domain
[ ]  915    0    0  915 / 1000  1375.6s Lin DSL Out_channel test with Domain
[ ]  945    0    0  945 / 1000  1490.0s Lin DSL Out_channel test with Domain
[✗] 1000    0    0 1000 / 1000  1519.5s Lin DSL Out_channel test with Domain

[...]

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

Test Lin DSL Out_channel test with Domain failed:

Negative test Lin DSL Out_channel test with Domain succeeded but was expected to fail
================================================================================
failure (1 tests failed, 0 tests errored, ran 2 tests)

and here's a recent Linux arm64 failure to trigger on 5.1:
https://ocaml-multicoretests.ci.dev:8100/job/2023-06-05/132328-ci-ocluster-build-4724e4

random seed: 255390210
generated error fail pass / total     time test name

[ ]    0    0    0    0 / 1000     0.0s Lin DSL In_channel test with Domain
[ ]    0    0    0    0 / 1000     0.0s Lin DSL In_channel test with Domain (generating)
[✓]    4    0    1    3 / 1000     5.6s Lin DSL In_channel test with Domain

[ ]    0    0    0    0 / 1000     0.0s Lin DSL Out_channel test with Domain
[ ]   61    0    0   61 / 1000    63.4s Lin DSL Out_channel test with Domain
[ ]  219    0    0  219 / 1000   212.7s Lin DSL Out_channel test with Domain
[ ]  560    0    0  560 / 1000   273.9s Lin DSL Out_channel test with Domain
[ ]  746    0    0  746 / 1000   334.4s Lin DSL Out_channel test with Domain
[ ]  858    0    0  858 / 1000   395.4s Lin DSL Out_channel test with Domain
[✗] 1000    0    0 1000 / 1000   418.3s Lin DSL Out_channel test with Domain

[...]

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

Test Lin DSL Out_channel test with Domain failed:

Negative test Lin DSL Out_channel test with Domain succeeded but was expected to fail
================================================================================
failure (1 tests failed, 0 tests errored, ran 2 tests)

[jmid]

Fixed in #370