I need to test a launcher app icon on an iPhone. Is there any way to do that without having to install an entire app?
You could create an HTML page for this which refers to the icon.
<link rel="apple-touch-icon" href="/custom_icon.png"/>
If you then open the web page and add the site to your home screen it will take
custom_icon.png as your app icon.
For more detailed information have a look at Apple's developer page.
You can use tools for testing icon design such as http://www.ikonica.me, it's much quicker than creating an HTML file, especially if you have to test multiple designs at once.
You can also use http://testico.net. Nothing to download on your desktop, no app to install on your phone, free.