Decouple clustered e2e tests and migrate to their own directory

[mkcp]

Is your feature request related to a problem? Please describe.

Currently, our with-cluster e2e tests rely on 20_zarf_init_test.go to run and configure a cluster before they can run. This presents a few problems.

First, the zarf init test not only sets up a test cluster, it also runs assertions and test cases against it. This means it is both test setup and a test itself. This adds overhead and implicit ordering dependencies if you want to set up multiple clusters.

Describe the behavior you'd like

Decouple cluster initialization and zarf init behavioral tests from 20_zarf_init_test.go. Migrate all with-cluster tests into their own directory.

Additional context

• main_test.go still must run first.
• Just set up cluster from 20_zarf_init_test.go once and share it still.
• Running tests in serial is fine for this one, parallelization is specced out in parent issue Parrallelize e2e test suite #3335
• Tunable timeouts would be nice but not required. 35m feels arbitrary