This repo is designed as a simple boilerplate for having multiple polymer elements and Karma test runner.
polymer.html is loaded in the test/karma-loader.js, and after polymer-ready fires, mocha will start running.
The tests in main/components/a/a.test.js and main/components/b/b.test.js each manage adding their own link tags to the document in the before setup block. This setup is quick and dirty but you could probably create a function that takes an array of imports and loads them all into the head for you.
You have to listen for the load even from the link tag to know when it's ok to start testing.