test: Add more test coverage of `validate` command's interaction with `backend` blocks

[SarahFrench]

This is a sneaky follow-up to https://github.com/hashicorp/terraform/pull/37972, as I realised that I had another test case that I hadn't pushed to that branch. Also there are some refactoring and small fixes to test failure messages from the first PR.

Target Release

1.15.x

Rollback Plan

• [x] If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

CHANGELOG entry

• [ ] This change is user-facing and I added a changelog entry.
• [x] This change is not user-facing.