Add acceptance and internal tests for `gcs` backend, add some test helper functions

[SarahFrench]

Historically the gcs backend hasn't been touched much. In the past I've added a few features but largely left the existing codebase as it was previously. Recently the gcs backend was refactored in https://github.com/hashicorp/terraform/pull/34989 to avoid using the old SDK.

As part of reviewing that PR I started adding a few acceptance tests in this PR. There's a lot more work to be done but I want to avoid this PR getting too big.

This PR:

• Separates acceptance tests and 'internal' tests into separate _test.go files
• Implements some pre-check functions to test for TF_ACC being set and credentials being supplied by the test environment
  • When this info is missing the tests are skipped, to enable checks passing in this repo. This repo has no automated testing of the gcs backend set up (//TODO!)
• Adds internal tests that assert:
  • TestBackendConfig_encryptionKey - how the config and ENVs for customer-supplied encryption keys are used
  • TestBackendConfig_kmsKey - how the config and ENVs for customer-managed (KMS) encryption keys are used
• Adds acc tests the assert:
  • TestAccBackendConfig_credentials - empty strings in the backend config don't interrupt use of ENVs for creds.
  • Note: we cannot have tests where we assert a failure, due to legacy reasons.

Target Release

1.8.x

Draft CHANGELOG entry

NEW FEATURES | UPGRADE NOTES | ENHANCEMENTS | BUG FIXES | EXPERIMENTS

[SarahFrench]

Just rebased to include the latest gcs PR

[SarahFrench]

Tests pass when I run them with TF_ACC=1 locally, merging. Thanks for the review John!

[github-actions[bot]]

Reminder for the merging maintainer: if this is a user-visible change, please update the changelog on the appropriate release branch.

[github-actions[bot]]

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