core: Fix TestContext2Apply_provisionerInterpCount

This was relying on a no-longer-valid mechanism for accessing the "count"
value from a resource block. The original issue this test was written to
cover is not really such a sharp edge anymore, since the length is taken
from the state during apply rather than from configuration, but it's still
a good case to cover.

terraform/test-fixtures/apply-provisioner-interp-count/provisioner-interp-count.tf

@@ -3,15 +3,15 @@ variable "num" {
 }
 
 resource "aws_instance" "a" {
-  count = "${var.num}"
+  count = var.num
 }
 
 resource "aws_instance" "b" {
   provisioner "local-exec" {
-    # Since we're in a provisioner block here, this interpolation is
+    # Since we're in a provisioner block here, this expression is
     # resolved during the apply walk and so the resource count must
-    # be interpolated during that walk, even though apply walk doesn't
+    # be known during that walk, even though apply walk doesn't
     # do DynamicExpand.
-    command = "echo ${aws_instance.a.count}"
+    command = "echo ${length(aws_instance.a)}"
   }
 }