Use pytest for e2e tests

For end-to-end tests, the testing setup and logic is written into a CI job at [.github/workflows/e2e.yaml](https://github.com/containers/olot/blob/0f63a6c43b5cb6c327ab4742c48582135c844924/.github/workflows/e2e.yaml)

This could be changed so that e2e tests are done with pytest per this comment:
https://github.com/containers/olot/pull/131#discussion_r2556286174

Then it would still be run in the same ci job but it would just be a pytest invocation.

This could allow them to be run locally as part of development as needed.