chore: use ephemeral workspaces in tests

[mitchnielsen]

Summary

Closes https://github.com/PrefectHQ/terraform-provider-prefect/issues/218

To do

• [x] Add helpers for ephemeral workspaces
• [ ] Implement helpers for ephemeral workspaces where workspaces are referenced
• [ ] (Maybe in a separate PR) Ensure random names for resources, even where workspaces are not used, for better test isolation

[mitchnielsen]

@parkedwards - I got the ball rolling here and will pick it back up after the move 👍🏼

[parkedwards]

awesome - thank you @mitchnielsen