CI: add test-requirements to irrelevant-files

Starting with https://review.opendev.org/#/c/715469/ devstack no
longer installs test-requirements.

Change-Id: I4f58aa666093aea2ab50d916084453a4f66dac93

zuul.d/ironic-jobs.yaml

@@ -21,6 +21,7 @@
       - ^ironic/tests/.*$
       - ^releasenotes/.*$
       - ^setup.cfg$
+      - ^test-requirements.txt$
       - ^tools/.*$
       - ^tox.ini$
     vars: