terraform test: remove planned state from plan

[liamcervante]

This is an internal refactoring of the PlannedState field within the Terraform internal plan struct. This was added in the past to help the testing framework perform validations after a plan operation. As of 0f845ad this is no longer required. The PlannedState was only being referenced either in places it wasn't needed, or just within test files. There's no behaviour change here, just removing the no-longer used field to ensure it doesn't cause confusion elsewhere or become a dependency by accident that we'd have to maintain long term.

[apparentlymart]

Heh, funny coincidence that on Friday I was looking at this and wondering if it was actually still needed, but found references to it that I wasn't familiar with and so put the question aside. I'm glad you had a similar thought!

I'll let someone directly on the core team be the reviewer, but just wanted to say thanks for checking in on this and tidying up my loose end! 😀