Move lockfree tests from multicoretests repo here

[lyrm]

This PR :

• organizes test directory in sub-directories so each (future) data structure has it own test directory,
• moves the tests for ws_deque from multicoretests repo to here.

[jmid]

I am (not surprisingly) supportive of moving the relevant multicoretests of lockfree here! 😃

However I think it may be a bit too premature to do so ATM.
For one, just yesterday I was wrapping up a QCheck PR c-cube/qcheck#244 for Test.make_neg which will eliminate the clunky check_error_count and simplify the dune files (see ocaml-multicore/multicoretests#78).
For another, I am still not happy with the amount of code required for situations such as this that require

• different cmd generators
• more than two parallel domains.

Unfortunately I haven't gotten around to improve the situation (yet) 🤷‍♂️ 😬

[lyrm]

@jmid : Thanks and sorry, I should have tagged you in my first message.

I totally can see your point, but in an other hand, it feels quite weird to have such important tests in another repository. But for now, I can cancel this PR, and make one for the lf_queue tests in multicoretests (I was planning to make another PR here with lf_queue and its tests after this one have been merged).

[lyrm]

Replaced by #43 .