Disable cmd shrinking for int64 ref Thread test

We are seeing long src/neg_tests/lin_tests_thread_ref.exe causing timeouts and long runs, primarily on Cygwin part 2 (examples below).

Since the experimental Lin Thread mode does not produce test failures as consistently as the Domain mode, this little PR disables cmd argument shrinking for the test, to limit how many shrinking candidates we have to traverse.

I've run focused tests which shows runs limited to at most 771.6s on Cygwin with the fix:

https://github.com/ocaml-multicore/multicoretests/actions/runs/6066711863/job/16457611765 5.1
https://github.com/ocaml-multicore/multicoretests/actions/runs/6066711878/job/16457612074 trunk Interesting observation unrelated to this PR: the first (positive test) goes from 600-800s on 5.1 to about 1200s on trunk (which causes the former not to complete 20 iterations before the 6h timeout)

Examples of timeouts and long runs:

https://github.com/ocaml-multicore/multicoretests/actions/runs/6049285277/job/16426696553 timeout during 9221.8s Lin ref int64 test with Thread (shrinking: 22.0013)
https://github.com/ocaml-multicore/multicoretests/actions/runs/6042037294/job/16404453112 long run 5144.7s Lin ref int64 test with Thread
https://github.com/ocaml-multicore/multicoretests/actions/runs/6026531927/job/16358872865 timeout during 11039.8s Lin ref int64 test with Thread (shrinking: 50.0002)
https://github.com/ocaml-multicore/multicoretests/actions/runs/6026072299/job/16356758666 long run 2381.5s Lin ref int64 test with Thread
https://github.com/ocaml-multicore/multicoretests/actions/runs/6026072300/job/16358585587 timeout during 8343.3s Lin ref int64 test with Thread (shrinking: 38.0002)
https://github.com/ocaml-multicore/multicoretests/actions/runs/6025647317/job/16355293724 long run 3214.6s Lin ref int64 test with Thread

ocaml-multicore / multicoretests

Disable cmd shrinking for int64 ref Thread test #389