Extend STM Array tests

[jmid]

I started going over our existing tests, and discovered a couple of Array entries which could easily be targeted with STM:

• for_all
• exists
• find_opt
• find_index
• stable_sort
• fast_sort

Afterwards, when the 5.0 CI workflows couldn't compile, I realized that Array.find_index was a recent 5.1 addition.
For now, this PR therefore leaves it commented out, and so leaves us extending with 5/6 of the above.

Handling an entry conditionally will require some machinery and potentially add additional dependencies.
I don't think it is warranted to do so ATM just to target find_index... 🤷

[jmid]

CI summary:

• freebsd-amd64-5.1 failed to find a counterexample in 6888.7s Lin Out_channel test with Domain Lin Out_channel test fails to trigger on FreeBSD #401

Out of 63 workflows 1 failed - a false alarm

[jmid]

CI summary for merge to main

• Bytecode 5.1 failed by finding an unexpected counterexample to Lin.Internal CList int test with Thread Lin.Internal Thread int CList test finds unexpected counterexample under bytecode #358

Out of 38 workflows 1 failed with a false alarm