Thread joingraph unification (chore)

[jmid]

The PR rewrites the two separate Atomic and tak tests inside src/thread/thread_joingraph into one.
They were written, quick-and-dirty, over 1 year ago and large left untouched (see the git history).

This is a step toward bringing them closer to the style of the effective src/threadomain test.
As part of the rewrite it now occasionally triggers an explicit Gc.minor call.

The rewrite will make it easier to later experiment with and add additional functionality (signal raising, finalizers, ...)
If nothing else, it will also shave a couple of seconds of the test suite run time.

[jmid]

CI summary:

• Cygwin trunk/5.2 part1 - timeout during a 4528.5s Lin DSL Out_channel test with Domain shrinking run
• Windows 5.1 - timeout due to a 11331.5s Lin DSL Out_channel test with Domain shrinking run
• 2 x Windows trunk/5.2 - timeout during threadomain [ocaml5-issue] Windows failures on threadomain #203

I'll go ahead and merge