tox.ini: Fix py3 environment to use packaged tests

Related D2082

tox.ini

@@ -6,7 +6,9 @@ deps =
   .[testing]
   pytest-cov
 commands =
-  pytest --cov=swh --cov-branch {posargs}
+  pytest --cov={envsitepackagesdir}/swh/model \
+         {envsitepackagesdir}/swh/model \
+           --cov-branch {posargs}
 
 [testenv:flake8]
 skip_install = true