Allow tox test to take posargs

Signed-off-by: Luiz Carvalho <lucarval@redhat.com>

tox.ini

@@ -6,7 +6,7 @@ deps =
     courier_master: -e git://github.com/operator-framework/operator-courier#egg=operatorcourier
     courier_released: operator-courier
     .[test]
-commands = pytest --cov=omps --ignore=tests/integration tests/
+commands = pytest --cov=omps --ignore=tests/integration {posargs:tests/}
 
 [coverage:report]
 fail_under = 90