FIX: corrects a regression hiding avatar in user selector (#20107)

Due to the way templates work, the incorrect variable (user instead of item) was not causing any error, and just failing silently to display the avatar.

This commit is also providing a basic spec for completion of users and groups.

app/assets/javascripts/discourse/app/templates/user-selector-autocomplete.hbr

@@ -4,7 +4,7 @@
       {{#if item.isUser}}
         <li>
           <a href title="{{item.name}}" class="{{item.cssClasses}}">
-            {{avatar user imageSize="tiny"}}
+            {{avatar item imageSize="tiny"}}
             <span class='username'>{{format-username item.username}}</span>
             {{#if item.name}}
               <span class='name'>{{item.name}}</span>