Introduce a “weight“ to #slow tests

Problem/Motivation

Follow up to #3497431: Deprecate TestDiscovery test file scanning, use PHPUnit API instead

@mondrake #12

I was thinking in a follow up we could introduce a “weight“ to #slow (i.e. #slow300, #slow200, etc) and further tune the start sequence.

@catch #13

[...] I think at the moment this is OK because we have less #slow tests than we run tests concurrently per-job (e.g. we might have 5 slow tests but start 8 at the same time), however it took a while for that to be the case and it might not be the case again, so definitely worth an issue. Even now it might save a few seconds which is worth having.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet