Update tests readme reference

[emmanuel-ferdman]

Summary

Commit cebf9176a3108317ea7d3f39f40a693dc721ff28 moved the location of the tests/README.md. This PR adjusts sources to changes.

Checklist

• [ ] If code changes were made then they have been tested.
  • [ ] I have updated the documentation to reflect the changes.
• [ ] This PR fixes an issue.
• [ ] This PR adds something new (e.g. new method or parameters).
  • [ ] This change has an associated test.
• [ ] This PR is a breaking change (e.g. methods or parameters removed/renamed)
• [x] This PR is not a code change (e.g. documentation, README, ...)