Fix test cleanup after creating temporary admin user (#13982)

This fixes a test failure due to recent bugfix to prevent deletion
of immutable users. The test in question manually edits the
account.bsdusers table to allow testing logic related to case when
root account is disabled, but then uses user.delete to remove the
temporary admin account. The fix is to use datastore plugin to
directly delete the new admin user.

tests/api2/test_user_admin.py

@@ -29,7 +29,8 @@ def admin():
         try:
             yield admin
         finally:
-            call("user.delete", admin["id"])
+            call("datastore.delete", "account.bsdusers", admin["id"])
+            call("etc.generate", "user")
 
 
 def test_installer_admin_has_local_administrator_privilege(admin):