Increasing test count of STM int64 ref Thread test

[jmid]

We are seeing occasional failures to trigger a STM int64 ref Thread negative test, e.g., here on macOS ARM64 5.0:
https://ocaml-multicoretests.ci.dev:8100/job/2023-09-04/192503-ci-ocluster-build-e5c445#L1466

random seed: 409068930
generated error fail pass / total     time test name

[ ]    0    0    0    0 /  250     0.0s STM int ref test with Thread
[ ]    0    0    0    0 /  250     0.0s STM int ref test with Thread (generating)
[✓]  250    0    0  250 /  250     2.9s STM int ref test with Thread

[ ]    0    0    0    0 / 1000     0.0s STM int64 ref test with Thread
[✗] 1000    0    0 1000 / 1000    11.2s STM int64 ref test with Thread

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

Test STM int64 ref test with Thread failed:

Negative test STM int64 ref test with Thread succeeded but was expected to fail

This little PR therefore raises the count from 1000 to 2500.

[jmid]

CI summary:

• Cygwin part 1 trunk/5.2 Dynlink timeout [ocaml5-issue] Deadlock in Dynlink test on Cygwin+MinGW+MSVC #307
• MingW bytecode x 2 threadomain timeouts [ocaml5-issue] Windows failures on threadomain #203
• macOS 5.1 failure to trigger STM int ref test parallel asymmetric in 5000 iterations 'STM _ ref test parallel asymmetric' failure to trigger #364
• linux-ppc64le-5.2 failed with sequential STM Array and Bytes SEGVs [ocaml5-issue] Crashes and hangs on ppc64 trunk/5.2 #380

Overall, that's 4 true positives and 1 false positive out of 55 (46 + 9) jobs.