tox.ini: fix passing {posargs} in several environments

CONTRIBUTING.rst claims the following:

   Or to only run tests in a particular test module on Python 3.6::

    $ tox -e py36 -- testing/test_config.py

But without this patch, this doesn't work: the arguments after -- are
ignored and all tests are run.

Author: wjt

tox.ini

@@ -18,7 +18,7 @@ envlist =
 
 [testenv]
 commands =
-    {env:_PYTEST_TOX_COVERAGE_RUN:} pytest --lsof
+    {env:_PYTEST_TOX_COVERAGE_RUN:} pytest --lsof {posargs}
     coverage: coverage combine
     coverage: coverage report
 passenv = USER USERNAME COVERAGE_* TRAVIS
@@ -41,7 +41,7 @@ deps =
     py27: mock
     nose
 commands =
-    pytest -n auto --runpytest=subprocess
+    pytest -n auto --runpytest=subprocess {posargs}
 
 
 [testenv:linting]
@@ -58,7 +58,7 @@ deps =
     hypothesis>=3.56
     {env:_PYTEST_TOX_EXTRA_DEP:}
 commands =
-    {env:_PYTEST_TOX_COVERAGE_RUN:} pytest -n auto
+    {env:_PYTEST_TOX_COVERAGE_RUN:} pytest -n auto {posargs}
 
 [testenv:py36-xdist]
 # NOTE: copied from above due to https://github.com/tox-dev/tox/issues/706.