Split testing libraries off in a separate repo

[jmid]

I'm starting to see more pain points that indicate we should consider splitting off the 3 testing libraries into a separate repo, e.g., ocaml-multicore/multicoretestlibs:

• Opam installation updates #346 now run an opam installation workflow on all changes - to prevent situations like Remove ppx generators #342 breaking the newly added 4.13 compatibility. The installation workflow is however also triggered on test suite changes in src/. Some of this can be handled by adding conditions on the workflows, but it would be clearer and simpler with separate repos.
• The developer-focused README instructions for running the test suite added in Document CI testing options #349 are intermixed with library-user instructions. Both of them would be clearer if separated in each their repo.

These add to the known arguments, such as being able to use an off-the-shelf ocaml-ci on the library-repo.