core: don't use functions in the input-interpolate-var fixture

Using functions here causes this to test more than it needs to, and
requires additional setup in the corresponding tests.

terraform/test-fixtures/input-interpolate-var/child/main.tf

@@ -1,5 +1,6 @@
-variable "list" { }
+variable "length" { }
+
 resource "template_file" "temp" {
-  count     = "${length(split(",", var.list))}"
+  count     = var.length
   template  = "foo"
 }

terraform/test-fixtures/input-interpolate-var/main.tf

@@ -2,6 +2,6 @@ module "source" {
   source = "./source"
 }
 module "child" {
-  source  = "./child"
-  list = "${module.source.list}"
+  source = "./child"
+  length = module.source.length
 }

terraform/test-fixtures/input-interpolate-var/source/main.tf

@@ -1,3 +1,3 @@
-output "list" {
-  value = "foo,bar,baz"
+output "length" {
+  value = 3
 }