stateless_check: Add 'ignore' option for fail_action

Check failures are still logged, this allows testing the check
before actually enforcing it (with fail_action reject or fail_action
quarantine).

config.go

@@ -167,7 +167,7 @@ func checkFailActionDirective(m *config.Map, node *config.Node) (interface{}, er
 	}
 
 	switch node.Args[0] {
-	case "reject", "quarantine":
+	case "reject", "quarantine", "ignore":
 		if len(node.Args) > 1 {
 			return nil, m.MatchErr("too many arguments")
 		}

maddy.conf.5.scd

@@ -921,6 +921,9 @@ implicitly so you use then without an explicit block definition.
 All modules in this list support the following directives:
 - debug
   Enable verbose logging for this module.
+- fail_action ignore
+  Ignore the check failure, useful for testing deployments to see
+  how useful it is.
 - fail_action reject
   Reject the message instead of quarantining it.
 - fail_action quarantine