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.

accounts/tests/test_admin_password.py

@@ -24,7 +24,7 @@ class GeneratedAdminPasswordTestCase(TestCase):
             plan = [(migration, False)]
         out = io.StringIO()
         env = {k: v for k, v in os.environ.items() if k not in ("ADMIN_PASSWORD", "ADMIN_USERNAME")}
-        with override_settings(BASE_DIR=Path(base_dir)), patch("sys.argv", ["manage.py", "migrate"]), patch.dict(
+        with override_settings(BASE_DIR=Path(base_dir)), patch.dict(
             os.environ, env, clear=True
         ), redirect_stdout(out):
             create_admin(sender=None, plan=plan, apps=apps)