Apply suggestion from @chalmerlowe

Author: chalmerlowe

noxfile.py

@@ -227,7 +227,7 @@ def unit(session, protobuf_implementation):
         "--cov-config=.coveragerc",
         "--cov-report=",
         "--cov-fail-under=0",
-        # os.path.join("tests", "unit"),
+        os.path.join("tests", "unit"),
         *session.posargs,
         env={
             "PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION": protobuf_implementation,