Fix import path for unit tests

wagtail/admin/static_src/wagtailadmin/app/draftail.entry.js

@@ -1,4 +1,4 @@
-import draftail from '../../../../../client/src/components/Draftail';
+import draftail from '../../../../../client/src/components/Draftail/index';
 
 // Expose as a global variable, for integration with other scripts.
 window.draftail = draftail;