Add gc tests

[jmid]

This PR adds initial tests of the Gc module - and in particular testing of Gc.compact.

Exercising Gc.counters already triggered a known issue locally, which has recently been fixed on trunk: ocaml/ocaml#13370 I therefore expect the parallel test to fail on 5.2.0 and earlier.

This can be extended (and improved) in many ways adding (big)arrays, Weak, Ephemeron, custom blocks, finalizers, and dynamic Gc_ctrl changes.

[jmid]

CI summary for ca8cf93:

• 11 5.2 workflows failed as expected due to the Gc test
  • 32bit 5.2 failed with a segfault
  • Bytecode aborted with Fatal error: allocation failure during minor GC
  • Cygwin 5.2 aborted with Fatal error: allocation failure during minor GC
  • FP 5.2 aborted with Fatal error: allocation failure during minor GC
  • Linux 5.2 aborted with Fatal error: allocation failure during minor GC
  • Linux 5.2 debug aborted with Fatal error: allocation failure during minor GC
  • MinGW 5.2 exited with code -1073741819
  • MinGW bytecode 5.2 exited with code -1073741819
  • macOS-ARM64 5.2 failed with a segfault
  • macOS-intel 5.2 got signal BUS
  • freebsd-amd64-5.2 failed with a segfault
• 3 Cygwin/MinGW trunk workflows failed with a version mismatch after the 5.3 branching
  • Cygwin trunk
  • MinGW bytecode trunk
  • MinGW trunk
• Linux trunk debug failed with runtime/shared_heap.c; line 1392 ### Assertion failed: local->stats.pool_live_words == pool_stats.live [ocaml5-issue] Assertion failure on live words in GC test #470

Out of 33 workflows 15 failed with 12 genuine issues and 3 ci-setup issues

[jmid]

I removed the stress test, as the positive parallel test is already stress testing.
I've also rebased based on #471 which should take care of the MinGW+Cygwin version mismatching.

I still expect the 5.2 workflows to trigger ocaml/ocaml#13370 (unsure whether the cmd should be disabled under 5.2 testing 🤔 )

[jmid]

CI summary for 9ef90b5:

• 7 5.2 workflows failed as expected due to the Gc test
  • 32bit 5.2 failed the parallel STM Gc test with Allocated_bytes : -7.47499188671e+196
  • Bytecode 5.2 aborted with Fatal error: allocation failure during minor GC
  • Cygwin 5.2 aborted with Fatal error: allocation failure during minor GC
  • FP 5.2 aborted with Fatal error: allocation failure during minor GC
  • MinGW 5.2 failed with Fatal error: allocation failure during minor GC
  • macOS-intel 5.2 failed with BUS error
  • linux-s390x-5.2 - aborted with Fatal error: allocation failure during minor GC
• 3 debug runtime runs failed with an assertion error:
  • Linux 5.2 debug aborted with runtime/shared_heap.c; line 1392 ### Assertion failed: local->stats.pool_live_words == pool_stats.live
  • Linux 5.3 debug failed with runtime/shared_heap.c; line 1392 ### Assertion failed: local->stats.pool_live_words == pool_stats.live
  • Linux trunk debug failed with runtime/shared_heap.c; line 1392 ### Assertion failed: local->stats.pool_live_words == pool_stats.live

Out of 45 workflows 10 failed, all with genuine issues

[jmid]

CI summary for 6b2669b

• 10 5.2 workflows failed as expected due to the Gc test [ocaml5-issue] Gc.counters unsafe root registration #474
  • 32bit 5.2 failed STM Gc test parallel with a negative allocated_bytes
  • Cygwin 5.2 aborted with Fatal error: allocation failure during minor GC
  • FP 5.2 aborted with Fatal error: allocation failure during minor GC
  • Linux 5.2 debug aborted with Fatal error: allocation failure during minor GC
  • MinGW bytecode 5.2 failed with Fatal error: allocation failure during minor GC
  • macOS-ARM64 5.2 failed with signal BUS
  • macOS-intel 5.2 failed with signal BUS
  • linux-arm64-5.2 aborted with Fatal error: allocation failure during minor GC
  • linux-s390x-5.2 aborted with Fatal error: allocation failure during minor GC
  • freebsd-amd64-5.2 aborted with Fatal error: allocation failure during minor GC
• 2 debug runtime workflows failed with an assertion error [ocaml5-issue] Assertion failure on live words in GC test #470
  • Linux 5.3 debug failed with runtime/shared_heap.c; line 1392 ### Assertion failed: local->stats.pool_live_words == pool_stats.live
  • Linux trunk debug failed with runtime/shared_heap.c; line 1392 ### Assertion failed: local->stats.pool_live_words == pool_stats.live
• linux-ppc64le-5.2 found an unexpected STM Sys test parallel counterexample Unexpected STM Sys parallel test counterexample on s390x and ppc64le #466

Out of 45 workflows 13 failed with with 12 genuine issues and 1 false alarm