chore: run integration tests against Juju 4/beta

[dimaqq]

• adds matrix over Juju versions
• adds Juju channel 4/beta
• adds integration test timeout
• don't fail the overall job if 4/beta branch fails

[dimaqq]

Findings:

• Can't tests against Juju 4 on machines, because storage is not implemented.
• Heisenbug against Juju 3, stuck at 96%, killed by timeout.
• Heisenbug Bugs against Juju 4, stuck at 7%, killed by timeout.

I suspect that more tests were run than 7%, that it's just the amount of output we get.

[benhoyt]

The Juju 3 Heisenbug aside, I think it's probably too early to run these against Juju 4 on every PR. They'll just fail every time, and it'll be noise.

[dimaqq]

I agree.
Juju 4 vm not ready for this workload.
Juju 4 k8s could perhaps be used to figure out the tooling around Heisenbugs, but that would be a separate PR.