[D7] Test-only job shouldn't require constant rebases to detect which files were changed

Problem/Motivation

This is D7 backport of: #3401971: Test-only job shouldn't require constant rebases to detect which files were changed.

I think it is worth to change this for D7 as well, to have the same experience for contributors.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet