Open a new e10s window before each test

It should be easy to remove this block when Nightly goes all-e10s.
nit: not all of our unit tests are asynchronous, so some don't have the second argument, and don't call `done()`. this isn't an issue with any of the current tests here, but there is one in the [next batch of tests i intend to land](https://github.com/mozilla/addon-sdk/pull/1561) to this addon, and probably others to come later..