Fix test command in workflow to use direct Python invocation

Author: ahosgood

.github/workflows/_tests.yml

@@ -73,7 +73,7 @@ jobs:
       - name: Run Django checks
         run: poetry run python manage.py check
       - name: Run test suite
-        run: poetry run coverage run manage.py test etna
+        run: poetry run python manage.py test etna
       - name: Check for missing migrations
         run: poetry run python manage.py makemigrations --check --noinput