Fix pretty asserts in utilities files

[smola]

Description

Pytests transforms tests to convert calls to assert to richer pretty asserts. However, it does so only in files scanner for tests. That is, those matched by the test_*.py pattern.

This change adds _test_*.py to the list of valid test file patterns, and moves existing test utilities to use that convention and benefit from pretty asserts.

Pretty assertions include the values of variables used in assertions, diffs, etc.

Motivation

Workflow

1. ⚠️⚠️ Create your PR as draft
2. Follow the style guidelines of this project (See how to easily lint the code)
3. Work on you PR until the CI passes (if something not related to your task is failing, you can ignore it)
4. Mark it as ready for review

Once your PR is reviewed, you can merge it! ❤️

Reviewer checklist

• Check what scenarios are modified. If needed, add the relevant label (run-parametric-scenario, run-profiling-scenario...). If this PR modifies any system-tests internal, then add the run-all-scenarios label (more info).
• CI is green
  • If not, failing jobs are not related to this change (and you are 100% sure about this statement)
• if any of build-some-image label is present
  1. is the image labl have been updated ?
  2. just before merging, locally build and push the image to hub.docker.com