feat: Add config validation tests

Adds a test to validate that the Defender module returns an
error when an invalid responder type is specified in the
Caddyfile.  The test uses the caddytest package to assert
that the expected error is returned.

config_test.go

@@ -2,6 +2,7 @@ package caddydefender
 
 import (
 	"encoding/json"
+	"github.com/caddyserver/caddy/v2/caddytest"
 	"github.com/jasonlovesdoggo/caddy-defender/responders"
 	"testing"
 
@@ -186,3 +187,64 @@ func TestValidation(t *testing.T) {
 		require.ErrorContains(t, def.Validate(), "invalid IP range")
 	})
 }
+
+func TestDefenderValidation(t *testing.T) {
+	t.Run("Invalid responder type", func(t *testing.T) {
+		caddytest.AssertLoadError(t, `{
+  "admin": {
+    "disabled": true
+  },
+  "apps": {
+    "http": {
+      "servers": {
+        "srv0": {
+          "listen": [
+            "127.0.0.1:80",
+            "[::1]:80"
+          ],
+          "routes": [
+            {
+              "handle": [
+                {
+                  "handler": "defender",
+                  "ranges": [
+                    "localhost"
+                  ],
+                  "raw_responder": "pineapple"
+                },
+                {
+                  "body": "This is what a human sees",
+                  "handler": "static_response"
+                }
+              ]
+            }
+          ],
+          "automatic_https": {
+            "disable": true
+          }
+        },
+        "srv1": {
+          "listen": [
+            "127.0.0.1:83",
+            "[::1]:83"
+          ],
+          "routes": [
+            {
+              "handle": [
+                {
+                  "body": "Clear text HTTP",
+                  "handler": "static_response"
+                }
+              ]
+            }
+          ],
+          "automatic_https": {
+            "disable": true
+          }
+        }
+      }
+    }
+  }
+}`, "json", "unknown responder type")
+	})
+}