Fix oslo_debug_helper not running

Specify test directory so that tox won't complain
`ImportError: Start directory is not importable`.

Change-Id: I03108f71254e8cc99fb0fcab7d4f1f8614b4e60e
Closes-Bug: #1666560
diff --git a/tox.ini b/tox.ini
index b4953e7..fff568d 100644
--- a/tox.ini
+++ b/tox.ini
@@ -46,7 +46,7 @@
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:debug]
-commands = oslo_debug_helper {posargs}
+commands = oslo_debug_helper -t patrole_tempest_plugin/tests {posargs}
 
 [flake8]
 # E123, E125 skipped as they are invalid PEP-8.