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_read_cache.py

@@ -6,11 +6,6 @@ info and capabilities are read once per object."""
 import unittest
 from unittest.mock import MagicMock
 
-from django.conf import settings
-
-if not settings.configured:
-    settings.configure(MAC_OUI="52:54:10")
-
 from libvirt import libvirtError
 from vrtManager.instance import wvmInstance