Controller's render
method is different than view's one. You want to execute it in a view context:
@test = view_context.render 'spree/shared/footer'
The main difference between this method and render_to_string
is that this returns html_safe string, so html tags within your <%= @test %> won't be escaped.
UPDATE:
However this is not the proper way of dealing with your problem. You are looking for 'content_for'. This method allows you to build part of the view, which can be used within the layout. All you need to do is to modify your layout:
# Your application layout
<html>
<head>
...
</head>
<body>
yield :image
<div id="wrapper">
yield
</div>
</body>
</html>
Then within your view, you can specify what is to be displayed as yield :image
with
<% content_for :image do %>
<%# Everything here will be displayed outside of the wraper %>
<% end %>