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 on macOS 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 on macOS 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