Make testing cpp implementation optional

Signed-off-by: Dan O'Reilly <oreilldf@gmail.com>

python/tox.ini

@@ -10,7 +10,6 @@ setenv =
 commands =
     python setup.py -q --cpp_implementation develop
     python setup.py build_py
-    python setup.py -q test -q
-    python setup.py -q test -q --cpp_implementation
+    python setup.py -q test -q {posargs}
 deps =
     six