Fix issue in switch back test error messages changed in Terraform v1.6

SwitchBackToRemoteFuncError tests verify error messages, but the error
message for missing bucket key in the s3 backend differs depending on
the Terraform version.
Define a helper function to hide the difference.

tfmigrate/multi_state_migrator_test.go

@@ -1010,9 +1010,8 @@ resource "null_resource" "qux" {}
 		t.Fatalf("expected migrator plan error")
 	}
 
-	expected := "Error: \"bucket\": required field is not set"
-	if !strings.Contains(err.Error(), expected) {
-		t.Fatalf("expected migrator plan error to contain %s, got: %s", expected, err.Error())
+	if !containsBucketRequiredError(err) {
+		t.Fatalf("expected migrator plan error to contain bucket required error: %s", err.Error())
 	}
 }
 
@@ -1130,8 +1129,7 @@ resource "null_resource" "qux" {}
 		t.Fatalf("expected migrator plan error to contain %s, got: %s", expected, err.Error())
 	}
 
-	expected = "Error: \"bucket\": required field is not set"
-	if !strings.Contains(err.Error(), expected) {
-		t.Fatalf("expected migrator plan error to contain %s, got: %s", expected, err.Error())
+	if !containsBucketRequiredError(err) {
+		t.Fatalf("expected migrator plan error to contain bucket required error: %s", err.Error())
 	}
 }

tfmigrate/state_migrator_test.go

@@ -529,9 +529,8 @@ resource "null_resource" "bar" {}
 		t.Fatalf("expected migrator plan error")
 	}
 
-	expected := "Error: \"bucket\": required field is not set"
-	if !strings.Contains(err.Error(), expected) {
-		t.Fatalf("expected migrator plan error to contain %s, got: %s", expected, err.Error())
+	if !containsBucketRequiredError(err) {
+		t.Fatalf("expected migrator plan error to contain bucket required error: %s", err.Error())
 	}
 }
 
@@ -618,8 +617,7 @@ resource "null_resource" "bar" {}
 		t.Fatalf("expected migrator plan error to contain %s, got: %s", expected, err.Error())
 	}
 
-	expected = "Error: \"bucket\": required field is not set"
-	if !strings.Contains(err.Error(), expected) {
-		t.Fatalf("expected migrator plan error to contain %s, got: %s", expected, err.Error())
+	if !containsBucketRequiredError(err) {
+		t.Fatalf("expected migrator plan error to contain bucket required error: %s", err.Error())
 	}
 }

tfmigrate/test_helper.go

@@ -0,0 +1,38 @@
+package tfmigrate
+
+import "regexp"
+
+// SwitchBackToRemoteFuncError tests verify error messages, but the
+// error message for missing bucket key in the s3 backend differs
+// depending on the Terraform version.
+// Define a helper function to hide the difference.
+//
+// # Terraform v1.5
+//
+// ```
+// Error: "bucket": required field is not set
+// ```
+//
+// # Terraform v1.6
+//
+// ```
+//
+//	Error: Missing Required Value
+//
+//	  on main.tf line 4, in terraform:
+//	   4:   backend "s3" {
+//
+//	The attribute "bucket" is required by the backend.
+//
+//	Refer to the backend documentation for additional information which
+//	attributes are required.
+//
+// ```
+const testBucketRequiredErrorLegacyTF = `Error: "bucket": required field is not set`
+const testBucketRequiredErrorTF16 = `The attribute "bucket" is required by the backend`
+
+var testBucketRequiredErrorRE = regexp.MustCompile(testBucketRequiredErrorLegacyTF + `|` + testBucketRequiredErrorTF16)
+
+func containsBucketRequiredError(err error) bool {
+	return testBucketRequiredErrorRE.MatchString(err.Error())
+}