ref: convert incidentActivity.js to typescript (#54958)

fixtures/js-stubs/incidentActivity.js → fixtures/js-stubs/incidentActivity.ts

@@ -1,10 +1,14 @@
+import {ActivityTypeDraft} from 'sentry/views/alerts/types';
+
 import {User} from './user';
 
-export function IncidentActivity(params = {}) {
+export function IncidentActivity(
+  params: Partial<ActivityTypeDraft> = {}
+): ActivityTypeDraft {
   return {
     comment: 'incident activity comment',
     type: 3,
-    dateCreated: new Date(),
+    dateCreated: '',
     user: User(),
     id: '123',
     incidentIdentifier: '999',