test: no seeded admin password, settings changed only for a test, no local libvirt by default

- create_admin had a branch for `"test" in sys.argv` that gave the admin
  the password "admin"; 17 places signed in with it, and failed silently
  with ADMIN_PASSWORD set in the environment. The tests now make their
  own superuser and force_login it, and the shipped code has no test
  branch: the test runner gives migrate a password (also when
  ADMIN_PASSWORD is set but empty), so no data/admin_password is written
  into the checkout.
- AccountsTestCase builds its rows in setUpTestData instead of
  setUpClass and deleting them by hand.
- accounts tests set OTP_ENABLED with override_settings instead of
  assigning settings (one assignment was never undone, so later tests in
  the process ran with OTP on), and undo their app_settings change. The
  OTP login class reloads the URLs again only after the setting is off.
- computes/tests/test_views.py reached the machine's own libvirt
  (localhost over the socket) when TEST_LIBVIRT_HOST was unset; its live
  tests now run only with TEST_LIBVIRT_HOST, like the other live tests,
  as the README says.

appsettings/tests/test_appsettings.py

@@ -1,7 +1,8 @@
 import re
 
+from django.contrib.auth import get_user_model
 from django.contrib.messages import get_messages
-from django.test import RequestFactory, TestCase
+from django.test import TestCase
 from django.urls import reverse
 
 from appsettings.models import AppSettings
@@ -10,7 +11,8 @@ from appsettings.settings import app_settings, get_settings
 
 class AppSettingsTestCase(TestCase):
     def setUp(self):
-        self.client.login(request=RequestFactory().get("/"), username="admin", password="admin")
+        self.admin = get_user_model().objects.create_superuser("tests-admin", "admin@example.com", "pw")
+        self.client.force_login(self.admin)
 
     def test_get_settings_populates_singleton(self):
         AppSettings.objects.update_or_create(