* Fix S3 backend test affected by making the Workspaces method return errors via diagnostics
* Address diagnostics comparison issues in test by ensuring expected diagnostics are defined in the context of the config they're triggered by
* Fix failing test case `TestBackendConfig_EC2MetadataEndpoint/envvar_invalid_mode` by making `diagnosticBase` struct comparable
* Add compile-time checks that diagnostic types fulfil interfaces
* Stop diagnosticBase implementing ComparableDiagnostic, re-add S3-specific comparer code to s3 package
* Update tests to use the S3-specific comparer again
* Fix test case missed in refactoring
* Update the backend.Backend interface to use diagnostics as return value from StateMgr method
* Fix calls to `Fatalf`
* Replace use of `testChdir` with `t.Chdir`
* Update tests to use temporary directories with copied content, instead of using directories in the repo directly.
* Remove stacks copy of testChdir function
This has been replaced with t.Chdir from the standard library
* Replace remaining simple usage of testChdir
* Update guidance for using `tempWorkingDir`
* Replace use of testChdir in a function reused in a single test
* Update comments to no longer recommend using testChdir
* Remove testChdir function!
