Also runs linter on tests folder in tox.ini (#552)

Note the `Makfile` is already doing so via the `lint` target.

Author: AndreMiras

{{cookiecutter.project_slug}}/tox.ini

@@ -11,7 +11,7 @@ python =
 [testenv:flake8]
 basepython = python
 deps = flake8
-commands = flake8 {{ cookiecutter.project_slug }}
+commands = flake8 {{ cookiecutter.project_slug }} tests
 
 [testenv]
 setenv =