Fixed parsing of error messages in tests

Slightly different error messages in Terraform v1.6 and OpenTofu v1.6.

tfmigrate/test_helper.go

@@ -4,12 +4,16 @@ 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.
+// depending on the Terraform version and OpenTofu version.
 // Define a helper function to hide the difference.
-const testBucketRequiredErrorLegacyTF = `Error: "bucket": required field is not set`
-const testBucketRequiredErrorTF16 = `The attribute "bucket" is required by the backend`
+const testBucketRequiredErrorLegacyTerraform = `Error: "bucket": required field is not set`
+const testBucketRequiredErrorTerraform16 = `The attribute "bucket" is required by the backend`
+const testBucketRequiredErrorOpenTofu16 = `The "bucket" attribute value must not be empty`
 
-var testBucketRequiredErrorRE = regexp.MustCompile(testBucketRequiredErrorLegacyTF + `|` + testBucketRequiredErrorTF16)
+var testBucketRequiredErrorRE = regexp.MustCompile(
+	testBucketRequiredErrorLegacyTerraform + `|` +
+		testBucketRequiredErrorTerraform16 + `|` +
+		testBucketRequiredErrorOpenTofu16)
 
 func containsBucketRequiredError(err error) bool {
 	return testBucketRequiredErrorRE.MatchString(err.Error())