test: remove code no test run executes

- 25 vrtManager test modules configured settings themselves when Django
  had none (MAC_OUI, and in test_network_ip a fake django module): under
  manage.py test settings are always configured, so it never ran.
- webvirtcloud/tests/test_common_tags.py tested no tag: it fetched an
  unknown URL, as test_error_pages and test_logging already do.
- .gitignore covers the parallel runs' test_db_N.sqlite3 files too.

vrtManager/tests/test_usage.py

@@ -7,11 +7,6 @@ import unittest
 from types import SimpleNamespace
 from unittest.mock import patch
 
-from django.conf import settings
-
-if not settings.configured:
-    settings.configure(MAC_OUI="52:54:10")
-
 from vrtManager.instance import USAGE_MAX_WINDOW, wvmInstance
 
 XML = """<domain><devices>