one test suite for multiple backends

[TomasTomecek]

• [ ] have a single integration test suite which runs for:
  • [ ] rootless podman
  • [ ] root podman
  • [ ] docker
• [ ] keep backend-specific tests separately, as we do now and document why they are separate

(implementation details: this could be achieved using pytest parametrization of a class)

@rpitonak suggested to create a new issue for this