tox: fix/improve posargs with pexpect factor(s)

Author: blueyed

tox.ini

@@ -76,7 +76,7 @@ deps =
     pexpect
     {env:_PYTEST_TOX_EXTRA_DEP:}
 commands =
-    {env:_PYTEST_TOX_COVERAGE_RUN:} pytest testing/test_pdb.py testing/test_terminal.py testing/test_unittest.py {posargs}
+    {env:_PYTEST_TOX_COVERAGE_RUN:} pytest {posargs:testing/test_pdb.py testing/test_terminal.py testing/test_unittest.py}
 
 [testenv:py36-pexpect]
 platform = {[testenv:py27-pexpect]platform}