WIP: e2e tests framework with pytest

[tobihans]

Fixes Issue

Closes #677

Changes proposed

The approach is to exploit testcontainers to run the opal server & client (as pytest fixtures), before tests start.

Currently, all necessary containers are configured but they're not yet working as intended.

• Configure all containers
• Fix opal client unable to connect to the server (ConnectionRefusedError - needs debugging)
• Fix statistics issue on opal server (currently, enabling it results in process failure)
• Provide basic http clients for server, client and OPA (as fixtures).

Check List (Check all the applicable boxes)

• I sign off on contributing this submission to open-source
• My code follows the code style of this project.
• This PR does not contain plagiarized content.
• The title of my pull request is a short description of the requested changes.

Screenshots

Note to reviewers

This is a WIP for now. I hope to expand on it to get a fully functional framework.

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