Add test-double or similar

[runspired]

We end up needing to double/stub lots of APIs in our tests in order to support browser, node, phantom and IE9. Instead of the ad-hoc way we've been doing this, we should add test-double, sinon or similar.

[hjdivad]

Seems okay. I'd prefer sinon over test-double unless there's some particular reason for the latter.

[stefanpenner]

My personal preference is test-double 🥇

[hjdivad]

We can totally use test double for this.

Me and @rwjblue can just mutter to ourselves when working on the tests 😉 .

[rwjblue]

TBH, I probably would be muttering from the typescript anyways....

[runspired]

mutters...