[Tests] Pass command-lin options of tox to pytest

tox.ini

@@ -6,7 +6,7 @@ envlist = default
 whitelist_externals = poetry
     pytest
 skip_install = true
-commands = pytest --pylama --cov=biscuit biscuit/core/
+commands = pytest --pylama --cov=biscuit {posargs} biscuit/core/
 setenv =
     TEST_SCREENSHOT_PATH = {env:TEST_SCREENSHOT_PATH:}
     TEST_SELENIUM_HUB = {env:TEST_SELENIUM_HUB:}