Fix posargs for cov, diffcov

Author: jaraco

tox.ini

@@ -7,7 +7,7 @@ skip_missing_interpreters = True
 [testenv]
 commands =
     !cov,!diffcov: python -m unittest {posargs:discover}
-    cov,diffcov: python -m coverage run {[coverage]rc} -m unittest {posargs:posargs}
+    cov,diffcov: python -m coverage run {[coverage]rc} -m unittest {posargs:discover}
     cov,diffcov: python -m coverage combine {[coverage]rc}
     cov: python -m coverage html {[coverage]rc}
     cov: python -m coverage xml {[coverage]rc}