add failing test case

hcl2template/testdata/variables/validation/invalid_default.pkr.hcl

@@ -0,0 +1,9 @@
+
+variable "image_id" {
+  type    = string
+  default = "potato"
+  validation {
+    condition     = length(var.image_id) > 4 && substr(var.image_id, 0, 4) == "ami-"
+    error_message = "The image_id value must be a valid AMI id, starting with \"ami-\"."
+  }
+}