Testing: replace evergreen workspace with ephemeral workspaces

[mitchnielsen]

Summary

We currently use the github-ci-tests workspace in staging for Terraform Provider tests.

We should instead use ephemeral workspaces so we can avoid any potential errors / resource conflicts between CI runs.

See here for an example where ephemeral workspaces are used.

Acceptance criteria

• All tests use an ephemeral workspace
  • Part 1: chore: use ephemeral workspaces in tests #219
  • Part 2: TBD
• Ensure that ephemeral workspaces are deleted after success or if they fail partway through (look into sweepers)
• Existing/dangling workspaces are deleted
• Thought: ensure random names for resources, even where workspaces are not used, for better test isolation