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/apps.py

@@ -66,7 +66,6 @@ def create_admin(sender, **kwargs):
     """
     import os
     import secrets
-    import sys
     from django.contrib.auth.models import User
 
     from accounts.models import UserAttributes
@@ -82,17 +81,13 @@ def create_admin(sender, **kwargs):
     if User.objects.exists():
         return
 
-    is_testing = "test" in sys.argv
     admin_user = os.environ.get("ADMIN_USERNAME", "admin")
     admin_pass = os.environ.get("ADMIN_PASSWORD")
-    generated_password = not admin_pass and not is_testing
+    generated_password = not admin_pass
 
-    if not admin_pass:
-        if is_testing:
-            admin_pass = "admin"
-        else:
-            admin_pass = secrets.token_urlsafe(16)
-            _store_generated_password(admin_pass)
+    if generated_password:
+        admin_pass = secrets.token_urlsafe(16)
+        _store_generated_password(admin_pass)
 
     print(f"\033[1m* \033[92mCreating default admin user '{admin_user}'\033[0m")
     with transaction.atomic():