Extend STM and Lin to generate tests with 3 parallel domains

[jmid]

Some parallelism bugs may only show up with 3 parallel processes, see, e.g., Sec.4 of "Experiences with QuickCheck: Testing the Hard Stuff and Staying Sane" https://www.cs.tufts.edu/~nr/cs257/archive/john-hughes/quviq-testing.pdf

To have a chance of finding such bugs STM and Lin should be extended from the current 2 Domain.spawns.

This will most likely be an API breaking.

I made a quick proof-of-concept experiment some time back in the trifork-attempt branch: https://github.com/ocaml-multicore/multicoretests/tree/trifork-attempt