Specify where the unit tests are.

tox.ini

@@ -31,7 +31,7 @@ deps=
 sitepackages = False
 recreate = False
 commands =
-    unit: pytest -rsx -xv --cov=celery --cov-report=xml --cov-report term
+    unit: pytest -rsx -xv --cov=celery --cov-report=xml --cov-report term t/unit
     integration: pytest -rsx -xsv t/integration
 setenv =
     WORKER_LOGLEVEL = INFO