CI: Split Tezt tests job

Context

In some MRs (like !2447 (closed)) the job tezt:main now runs out of time (1 hour).

This MR first makes a couple of changes to make the tezt executable more scriptable, and then adds a script to easily split the job into smaller jobs.

As of now, there are 102 distinct tests, so I split into 3 jobs of about 33.

Manually testing the MR

Go look at the tezt CI jobs; check the 3 together run all the tests.

Checklist

• N/A Document the interface of any function added or modified (see the coding guidelines)
• Provide automatic testing (see the testing guide).
• N/A For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.