Enable helm pre-release test in TC (#1197)

Test helm provider with the latest change before release.

Author: dak1n1

kubernetes/test-infra/external-providers/.terraformrc

@@ -1,7 +1,10 @@
 provider_installation {
   filesystem_mirror {
     path    = "/tmp/.terraform.d"
-    include = ["localhost/test/kubernetes"]
+    include = [
+      "localhost/test/kubernetes",
+      "localhost/test/helm",
+    ]
   }
   direct {
     include = ["registry.terraform.io/*/*"]