Mock iam_endpoint explicitly for testing with localstack

Starting from Terraform v1.6, the s3 backend configuration has changed,
and the tfmigrate acceptance test fails. I suspect that the
authentication mechanism may have been changed with the migration to
aws-sdk-go-v2; I have tried a few things and found that I can
authenticate by specifying the `iam_endpoint`. The `iam_endpoint`
argument has been deprecated since Terraformv1.6. It is recommended to
use the newly added `endpoints` argument, though, for running tests on
multiple Terraform versions, the test settings must remain compatible
with the older versions. This is a test-only issue, so I'll use the
deprecated `iam_endpoint` for now. In the future, we should change the
backend configuration depending on the Terraform version.

test-fixtures/backend_s3/config.tf

@@ -5,8 +5,9 @@ terraform {
     bucket = "tfstate-test"
     key    = "test/terraform.tfstate"
 
-    # mock s3 endpoint with localstack
+    # mock s3/iam endpoint with localstack
     endpoint                    = "http://localstack:4566"
+    iam_endpoint                = "http://localstack:4566"
     access_key                  = "dummy"
     secret_key                  = "dummy"
     skip_credentials_validation = true

test-fixtures/storage_gcs/config.tf

@@ -5,8 +5,9 @@ terraform {
     bucket = "tfstate-test"
     key    = "test/terraform.tfstate"
 
-    # mock s3 endpoint with localstack
+    # mock s3/iam endpoint with localstack
     endpoint                    = "http://localstack:4566"
+    iam_endpoint                = "http://localstack:4566"
     access_key                  = "dummy"
     secret_key                  = "dummy"
     skip_credentials_validation = true

test-fixtures/storage_s3/config.tf

@@ -5,8 +5,9 @@ terraform {
     bucket = "tfstate-test"
     key    = "test/terraform.tfstate"
 
-    # mock s3 endpoint with localstack
+    # mock s3/iam endpoint with localstack
     endpoint                    = "http://localstack:4566"
+    iam_endpoint                = "http://localstack:4566"
     access_key                  = "dummy"
     secret_key                  = "dummy"
     skip_credentials_validation = true

tfexec/terraform_test.go

@@ -188,16 +188,17 @@ terraform {
     # bucket = "tfstate-test"
     key    = "%s/terraform.tfstate"
 
-    # mock s3 endpoint with localstack
+    # mock s3/iam endpoint with localstack
     endpoint                    = "%s"
+    iam_endpoint                = "%s"
     access_key                  = "dummy"
     secret_key                  = "dummy"
     skip_credentials_validation = true
     skip_metadata_api_check     = true
     force_path_style            = true
   }
 }
-`, t.Name(), endpoint)
+`, t.Name(), endpoint, endpoint)
 	source := `
 resource "null_resource" "foo" {}
 resource "null_resource" "bar" {}

tfexec/test_helper.go

@@ -271,16 +271,17 @@ terraform {
     bucket = "%s"
     key    = "%s"
 
-    # mock s3 endpoint with localstack
+    # mock s3/iam endpoint with localstack
     endpoint                    = "%s"
+    iam_endpoint                = "%s"
     access_key                  = "%s"
     secret_key                  = "%s"
     skip_credentials_validation = true
     skip_metadata_api_check     = true
     force_path_style            = true
   }
 }
-`, TestS3Region, TestS3Bucket, key, endpoint, TestS3AccessKey, TestS3SecretKey)
+`, TestS3Region, TestS3Bucket, key, endpoint, endpoint, TestS3AccessKey, TestS3SecretKey)
 	return backendConfig
 }