Add test coverage for native fallback

[alancutter]

We currently test using the *.dev.js versions of the polyfill. These do not include native fallback logic and as a consequence bugs relating to native fallback persist undetected until they are hit by users in the next release.
See: #123

[alancutter]

I made an error here, we currently run the web-platform-tests suite with the fallback disabled by manually hiding the native implementation:
https://github.com/web-animations/web-animations-js/blob/dev/test/resources/testharnessreport.js#L412

The *.dev.js versions of the polyfill are not used for automated testing.