testing: move internal end-to-end testing to public repo

[rjeczalik]

Currently terraform-provider-scylladbcloud is end-to-end tested internally, which does not help for accepting external contribution.

There are still a few prerequisites to be taken care of first, before it is possible to move those end-to-end tests here.

This issue is here to keep track of the effort, having real end-to-end tests definitely here definitely will increase confidence in accepting external contributions.