Use {posargs} for Tox test command target

This allows specifying a subset of tests to run from the Tox command
line, when developing.

Author: PiDelport

tox.ini

@@ -4,7 +4,7 @@ envlist =
     py{27,34,35,36}-dj1.{8,9,10}
 
 [testenv]
-commands=python {toxinidir}/test_project/manage.py test test_app --noinput
+commands=python {toxinidir}/test_project/manage.py test {posargs:test_app} --noinput
 passenv = DJANGO_VERSION POSTGRES_USER POSTGRES_PASSWORD TRAVIS_BRANCH
           TRAVIS_COMMIT TRAVIS_LOG_LEVEL TRAVIS_PULL_REQUEST_BRANCH
           TRAVIS_REPO_SLUG