Switch to using web-component-tester

@nevir To get a little experience using web-component-tester, I revived your PR to add support to seed-element for it and made the following changes:

* Bumped and tested with WCT 0.6.2 (the previous PR was several versions behind)
* Cleaned up some extra spacing we no longer needed
* Corrected the ignore for node_modules (it was previously using /node_modules, whilst it ignores fine in the shorter form)

Something to consider:

The tests directory is currently ignored in bower.json. Should we do the same for the gulpfile.js & package.json files or was keeping them a part of the package intentional?
My understanding is that only one `test()` can be passed `done()`. When I tried using multiple `done()`s in separate tests, I got `Error: done() called multiple times at multiple` errors. @nevir may know more about the limitations of the WCT setup.