Allow to only run unit tests with Tox.

tox.ini

@@ -15,7 +15,7 @@ install_command =
 commands_pre =
     find {toxinidir} -type f -not -path '{toxinidir}/.tox/*' -path '*/__pycache__/*' -name '*.py[c|o]' -delete
 commands =
-    pytest
+    pytest {posargs}
     py36,py37: black --check .
     isort --recursive --check-only --settings-path setup.cfg .
     flake8 .
@@ -25,6 +25,10 @@ basepython = python3.7
 commands =
     black {posargs} .
 
+[testenv:test]
+commands =
+    pytest {posargs}
+
 [testenv:end-to-end]
 deps = -rtest_requirements.txt
 commands =