MM-64316 Fix icon_emoji not working on webhook posts (#31068)

* MM-64316 Fix icon_emoji not working on webhook posts

* Add test case for custom emojis

* Remove unneeded test cleanup

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/app/webhook.go

@@ -344,7 +344,7 @@ func (a *App) CreateWebhookPost(c request.CTX, userID string, channel *model.Cha
 			post.AddProp(model.PostPropsOverrideIconURL, overrideIconURL)
 		}
 		if overrideIconEmoji != "" {
-			post.AddProp(model.PostPropsOverrideIconURL, overrideIconEmoji)
+			post.AddProp(model.PostPropsOverrideIconEmoji, overrideIconEmoji)
 		}
 	}