[CI] Introduce new pytest-based system test framework for stress test

Description

This MR:

• Adds a new framework for the system tests based on pytest.
• Adds a completely revamped stress test written from scratch in this new framework.
• The new system tests can be found under continuousintegration/system_tests.

The plan is to eventually move all system tests to this new framework.

Checklist

• Documentation reflects the changes made.
• Merge Request title is clear, concise, and suitable as a changelog entry. See our contributing docs

References

Closes #1350