Optimize test ordering when --group argument is passed to phpunit

Problem/Motivation

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We have various logic in run-tests.sh to optimize test ordering, however there are different code branches for different arguments, and it doesn't run when --group is passed in.

Unfortunately --group is the argument used by gitlab ci templates for contrib, so the ordering logic only works for core at the moment.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet