Test rework, part 2

[apostasie]

This is a first follow-up to #3418, rewriting more tests to leverage the new tooling.

In almost all cases, this is "identical" with the previous state of affairs, as these changes are not meant to introduce new tests, and normally do not radically change the existing tests intention.

However, here or in upcoming follow-ups:

• "requirements" have been precisely reviewed
  • some tests that were previously not "private" are now properly isolated
  • some tests that were previously not running for Docker (or other previously overstated requirements) will now run
• all tests are now parallelized by default

I appreciate this is hard to review. This is a full-on rewrite. But then, we are talking solely about tests here, with no code or logic change - and the proof is in the pudding: aka green CI.

I am splitting different subfolders tests in different commits. As I am converting more tests, I will add more commits to this branch.

We should still merge this PR whenever convenient (pending green of course) - and I will just open more PRs - that should help keep this bite-size and avoid conflicting with main.

[apostasie]

@AkihiroSuda let's merge this if these changes are fine with you. I am in testing hell in other PRs, and any improvement overall to the tests stability and isolation will help.

[apostasie]

Last push does rebase, and simplify completion-test to not flake.

[apostasie]

Failures with last push are the perennial IPFS/Compose, and testattach (current delay is definitely too short - will fix separately at 5 seconds which seems to do the trick).