[IndexedDB] Increase test converage

[TitanNano]

Come up with more tests for the IndexedDB driver to get the test coverage to 100%.

[opaldes]

Would like to write some test for the IndexedDB Driver, would probably need some help getting started. And some ressources on IDB I should read upon.

[TitanNano]

@opaldes IndexedDB is currently only covered by some tests for the ServiceWorker cache and doesn't have any tests of it's own. There is a indexedDB shim for tests though and you should be able to use it. You can consult the wiki and MDN on how the driver works and how the indexedDB API it self works.