tox: add positional arg handling

.travis.yml

@@ -30,7 +30,7 b' install:'
 - travis_retry pip install -U setuptools pip tox
 
 script:
-- travis_retry tox
+- travis_retry tox -- -vv
 
 services:
 - postgresql

tox.ini

@@ -12,4 +12,4 b' commands='
     pip install -r backend/requirements.txt
     pip install -e backend
     appenlight-reindex-elasticsearch -c testing.ini -t all
-    pytest backend/src/appenlight/tests
+    pytest backend/src/appenlight/tests {posargs}