Skip litmus tests for unit tests update

Ignore a few more paths used by unit tests to avoid executing
unnecessary jobs.

Change-Id: I6b20fdb5b586410d4af9f41727cd336e43139e10

zuul.d/litmus.yaml

@@ -16,6 +16,7 @@
       - ^spec/unit/.*$
       - ^spec/classes/.*$
       - ^spec/defines/.*$
+      - ^spec/type_aliases/.*$
       - ^requirements.txt$
       - ^test-requirements.txt$
       - ^metadata.json$