actually enable our no tempest/tests check

I'm stupid and forgot to enable this, actually turn it on.

Change-Id: I050334bd0f0e5e282e6485b52aaf1e74eed9c7c9
diff --git a/tempest/hacking/checks.py b/tempest/hacking/checks.py
index 93cf89d..5e941da 100644
--- a/tempest/hacking/checks.py
+++ b/tempest/hacking/checks.py
@@ -66,3 +66,4 @@
 def factory(register):
     register(skip_bugs)
     register(import_no_clients_in_api)
+    register(import_no_files_in_tests)