Execute acceptance tests in CI (GH actions) also against a clustered Incus setup

[breml]

Currently CI is executing the tests only against a non clustered Incus setup and therefore all the tests which require a clustered Incus setup to work (all tests having the acctest.PreCheckClustering(t) pre check) are currently never executed in CI.

[breml]

@maveonair With FuturFusion/operations-center#377 landed in Operations Center end of last week, we added some real end2end tests in Operations Center using some beefy selfhosted github runners, deploying Operations Center and 3 instances of Incus (all running on top of IncusOS) and then perform several test like actual clustering.

This code could also serve as foundation or blueprint to add some end2end tests with clustered Incus for the Terraform provider.