[RED] Add test for seed_users command

backend/main/test_commands.py

+from io import StringIO
+from django.core.management import call_command
+from django.test import TestCase
+
+from .models import User
+
+
+class SeedUserTest(TestCase):
+    def test_command_success(self):
+        out = StringIO()
+        users_amount = len(User.objects.filter())
+        call_command('seed_users', stdout=out)
+        self.assertIn('Successfully seeds 100 users.', out.getvalue())
+        self.assertEqual(users_amount + 100, len(User.objects.filter()))