Zombienet: PR vs prev release tests

[sandreim]

trafficstars

The test should ensure upgrading the node from prev release works as expected.

[rphmeier]

I think we'd want 50% master, 50% prev-release with a few parachains registered. And then maybe 50% of collators on master, 50% on prev-release. And then we'd have some health checks to make sure things are working as expected.