tests: Stricter playbook and inventory parsing

tests/ansible/ansible.cfg

@@ -10,6 +10,7 @@ callbacks_enabled =
 callback_whitelist =
     profile_tasks,
     timer
+duplicate_dict_key = error
 inventory = hosts
 gathering = explicit
 strategy_plugins = ../../ansible_mitogen/plugins/strategy
@@ -46,9 +47,13 @@ timeout = 30
 host_key_checking = False
 
 [inventory]
+# Fatal error if any inventory source is unparsed by every available plugin.
 any_unparsed_is_failed = true
+# Fatal error if no inventory sources have a match for a host pattern.
 host_pattern_mismatch = error
 ignore_extensions = ~, .bak, .disabled
+# Fatal error if no inventory sources are successfully parsed.
+unparsed_is_failed = true
 
 [callback_profile_tasks]
 task_output_limit = 10