Interoperability tests between multiple core versions

[Hocuri]

We would like to have some automated tests that test basic interoperability (scanning QR code, creating a group, sending a message) between current main and an older, fixed version of core.

This would be a script that:

• Creates two virtual environments
• In one of them, installs current main (see scripts/make-rpc-testenv.sh; in CI, the binary can be downloaded instead of building it, just like "JSON-RPC Python tests")
• In the other one, installs a fixed, older version from pypi
• Runs both of these cores, and does some basic tests.

The script can then be added to CI.

These tests could prevent problems like this one: #7384

[link2xt]

After some discussion with @hpk42 we have an update to the architecture of this.
It should be integarted into existing JSON-RPC python tests, with a pytest fixture (similar to acfactory) that can create a virtual environment, install deltachat-rpc-client and deltachat-rpc-server from PyPI in there and then run the test code inside of it using execnet.

execnet is already used by pytest-xdist and in chatmail relay deployment and testing code.

So we don't need to create two virtual environments, only a virtual environment for running old version of the code. New code should run in the same venv as pytest. We only need to test interoperability between the current version and some fixed old version as a first step.