core: Fix TestContext2Input_submoduleTriggersInvalidCount

Its fixture was using a module variable called "count", and that name is
now reserved.

terraform/test-fixtures/input-submodule-count/main.tf

@@ -1,4 +1,4 @@
 module "mod" {
   source         = "./mod"
-  count = 2
+  instance_count = 2
 }

terraform/test-fixtures/input-submodule-count/mod/main.tf

@@ -1,8 +1,8 @@
-variable "count" {
+variable "instance_count" {
 }
 
 resource "aws_instance" "foo" {
-  count = "${var.count}"
+  count = "${var.instance_count}"
 }
 
 module "submod" {