feat(tests): add Silence component tests

ui/src/setupTests.js

@@ -15,6 +15,9 @@ require("jest-localstorage-mock");
 // favico.js needs canvas
 require("jest-canvas-mock");
 
+// used to mock current time since we render moment.fromNow() in some places
+require("jest-date-mock");
+
 // fetch is used in multiple places to interact with Go backend
 // or upstream Alertmanager API
 global.fetch = require("jest-fetch-mock");