Call unittest directly from tox.ini (#1133)

Since moving to pyproject.toml tox began failing since the setup module had been removed. Instead we'll unittest directly from tox.
diff --git a/tox.ini b/tox.ini
index e2e67a6..3fcea7e 100644
--- a/tox.ini
+++ b/tox.ini
@@ -3,4 +3,4 @@
 
 [testenv]
 commands=
-    python setup.py test
+    python -m unittest discover -p '*_test.py' yapftests/