Fix diagnostic comparison issues in s3 backend tests

[SarahFrench]

This PR is stacked on #37508

Once this PR is merged all the S3 tests will be passing.

After diagnostic comparison became more stringent s3 tests were failing because some diagnostics had Source data associated with them, while the expected diagnostics in the test cases did not include that information. The Source data that causes the issue appears to be added here in a backend testing helper:

terraform/internal/backend/testing.go

Line 45 in 8f41cc8

diags = diags.Append(valDiags.InConfigBody(c, ""))

To address this, I've updated relevant test cases to similarly add config contect to diagnostics in some test cases.

Target Release

N/A

Rollback Plan

• 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.
• This change is not user-facing.

[SarahFrench]

When I run the s3 tests locally they all pass 🎉

go test github.com/hashicorp/terraform/internal/backend/remote-state/s3                   
ok      github.com/hashicorp/terraform/internal/backend/remote-state/s3 13.361s

[jar-b]

LGTM 🎉

Thanks for fixing these!

% TF_ACC=1 go test -count=1 ./...
ok      github.com/hashicorp/terraform/internal/backend/remote-state/s3 213.818s

The base branch was changed.

[SarahFrench]

I got some feedback about making diagnosticBase implement ComparableDiagnostic: diagnostic comparisons are used in Terraform to held deduplicate diagnostics that are shown to users. The diagnosticBase type has no source information so it's possible that two different diagnostics arising from different places could mistakenly be identified as equal, and therefore one will be removed from the deduplicated output shown to users.

Because of the potential risk affecting users seeing diagnostics, I've removed the Equal method from diagnosticBase implement, and I've restored the s3 tests to using their own package-specific comparer.

The only reason we needed to make diagnosticBase implement ComparableDiagnostic is because Sourceless diagnostics are returned from here:

terraform/internal/backend/remote-state/s3/backend.go

Lines 1088 to 1095 in 8b65426

	_ /* ctx */, awsConfig, cfgDiags := awsbase.GetAwsConfig(ctx, cfg)
	for _, d := range cfgDiags {
	diags = diags.Append(tfdiags.Sourceless(
	baseSeverityToTerraformSeverity(d.Severity()),
	d.Summary(),
	d.Detail(),
	))
	}

If the hashicorp/aws-sdk-go-base/v2 dependency was updated so that the diagnostic could be linked to places in the configuration then the diagnostics could stop being Sourceless and then would be compatible with the shared comparer logic. However that's a big ask, so updating this PR to return the s3-specific comparer to this package is the easiest solution.

[SarahFrench]

I ran the s3 tests locally and they're passing:

% go test github.com/hashicorp/terraform/internal/backend/remote-state/s3
ok      github.com/hashicorp/terraform/internal/backend/remote-state/s3 13.551s

[SarahFrench]

This is a lightly changed version of the original file removed in #36385