test: add mqtt config parsing coverage

tests/NATS.Server.Tests/TestData/mqtt.conf

@@ -0,0 +1,28 @@
+mqtt {
+    listen: "10.0.0.1:1883"
+    no_auth_user: "mqtt_default"
+
+    authorization {
+        user: "mqtt_user"
+        pass: "mqtt_pass"
+        token: "mqtt_token"
+        timeout: 3.0
+    }
+
+    tls {
+        cert_file: "/path/to/mqtt-cert.pem"
+        key_file: "/path/to/mqtt-key.pem"
+        ca_file: "/path/to/mqtt-ca.pem"
+        verify: true
+        timeout: 5.0
+    }
+
+    ack_wait: "60s"
+    max_ack_pending: 2048
+    js_domain: "mqtt-domain"
+    js_api_timeout: "10s"
+    stream_replicas: 3
+    consumer_replicas: 1
+    consumer_memory_storage: true
+    consumer_inactive_threshold: "5m"
+}