Allow running tests concurrently with watch server, again!

[ElectreAAS]

Contributed to #8114
Supersedes #11900.

This PR is a 'remake' of Steve's: I rebased it on main, and applied what I learned making #12010 and #12064. This results in significant changes from the original, hence the new PR.

The 'runtest' RPC request is now public, in otherlibs. runtest_rpc.ml{i} was removed.

The behaviour is now (AFAI know what I'm doing) identical between running dune runtest [ARGUMENTS] with or without the RPC server.

See comments below for details

[ElectreAAS]

Notes from rebasing: Since the contents of bin/runtest.ml have been largely moved to bin/runtest_common.ml, it'd be easy to accidentaly overwrite changes made to the former since the original PR.
#11869 and #12251 are PRs that fit that description, and have been correctly applied to runtest_common

[ElectreAAS]

Outdated

The last commit (27412bc) is maybe a bit overzealous in the refactoring, but that was the only way I found to make sure the error messages were correct.
Review can be done without it, and if deemed necessary I could put it in a different PR

Up to date

I just force-pushed a few changes:

• it's now rebased on main
• there are now only 2 commits:
  • the first is the one that has only what is necessary for the feature, without previous refactorings
  • the second has all the plumbing required to make the 'forwarding' warning appear at the correct time (after actually connecting to the RPC server)

It's now both ready to review and reviewable! 😃

PS: the previous state of this PR is still visible on my fork, as a backup