Move conftest.py into tests/ for proper pytest fixture discovery

[schoekek]

What does this pull request do?

Problem:

• conftest.py is in the project root, next to tests/
• pytest automatically discovers fixtures only in test directories or their parents within the test tree
• In isolated CI/build environments (pybuild, Salsa CI), fixtures like elasticapm_client and waiting_httpserver are not found

Proposed solution:

• Move conftest.py into the tests/ directory
• This makes pytest discover all fixtures correctly in all environments
• Fully compliant with pytest best practices, no changes to fixture logic

Related issues

Closes #2536

[cla-checker-service]

❌ Author of the following commits did not sign a Contributor Agreement:
0fdf350

Please, read and sign the above mentioned agreement if you want to contribute to this project

[github-actions]

🤖 GitHub comments

Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)