Issue #3446628 by ptmkenny: Link to module in admin UI and show tab for test form

firebase_php.links.menu.yml

+firebase_php.config:
+  title: 'Firebase PHP'
+  route_name: firebase_php.config
+  description: 'Configure Firebase PHP and send a test message'
+  parent: system.admin_config_services
+  weight: 99

firebase_php.links.task.yml

+firebase_php.config:
+  route_name: firebase_php.config
+  base_route: firebase_php.config
+  title: 'Settings'
+  parent: system.admin_config_services
+
+firebase_php.config.links:
+  route_name: firebase_php.send_test_message_form
+  base_route: firebase_php.config
+  title: 'Send a test message'
+