Reenable Gc.compact in Gc tests

[jmid]

This one-line PR is a left-over from #469. It was omitted from that one to move forward with the Gc tests.

I expect the PR to trigger the two outstanding Gc issues:

• [ocaml5-issue] Assertion failure on live words in GC test #470
• [ocaml5-issue] Segfault or hang in STM Gc stress test parallel #480

To avoid introducing noise in CI runs, it should be merged once these two have been resolved.

[jmid]

CI summary:

• Linux 5.3 debug aborted stm_tests_par_stress with an assertion error [ocaml5-issue] Assertion failure on live words in GC test #470
• Linux trunk debug aborted stm_tests_par_stress with an assertion error [ocaml5-issue] Assertion failure on live words in GC test #470
• macOS-ARM64 5.3 segfaulted in stm_tests_par_stress [ocaml5-issue] Segfault or hang in STM Gc stress test parallel #480

These were as expected.

Out of 33 workflows 3 failed with genuine issues.

[jmid]

Dummy push to trigger a fresh CI rerun with the fix of ocaml/ocaml#13090 merged

[jmid]

CI summary for c592e05

• Cygwin 5.3 timed out after 10691.4s shrinking in Lin Bytes test with Domain Timeout due to excessive shrinking in Lin Bytes test with Domain on Cygwin #520
• Cygwin trunk timed out after 9783.0s shrinking in Lin Bytes test with Domain Timeout due to excessive shrinking in Lin Bytes test with Domain on Cygwin #520
• Linux 5.3 debug failed with an assertion error in STM Gc stress test parallel [ocaml5-issue] Assertion failure on live words in GC test #470

The latter is expected as #470 has now been fixed on trunk but will continue to trigger on 5.3.

Out of 34 workflows, 1 failed with a genuine error and 2 with false alarms

[jmid]

CI summary for dcb978b:

• Linux 5.3 debug failed in STM Gc stress test parallel with an assertion error local->stats.pool_live_words == pool_stats.live [ocaml5-issue] Assertion failure on live words in GC test #470
• OPAM installation / build-and-test (ocaml-variants.5.4.0+trunk) failed to install dune.3.17.2 due to a trunk incompatibility

Out of 36 workflows 2 failed with genuine errors.

[jmid]

I've just

• rebased on main,
• added a guard to Gc.compact in 404dd17, and
• added the issue to the README list

[jmid]

CI summary for 0a43592

• Cygwin trunk timed out in Lin Bytes test with Thread Deadlock in Lin Bytes test with Thread on Cygwin #526

Out of 51 workflows 1 failed with a borderline defect/reliability issue

[jmid]

CI summary for merge to main:

• Cygwin 5.3 timed out after 9799.3s while shrinking in STM Dynarray test parallel (int) Timeout due to excessive shrinking in STM Dynarray test parallel #554

Out of 51 workflows, 1 failed with a borderline defect/reliability issue