Stub out L.mapbox.map when testing.
Stub out L.mapbox.map when testing.

I'm not a fan of this fix, and it won't scale well as we build out
the Map component, but it makes the browser tests work for now.

mmmavis/teach.mozilla.org,mozilla/learning.mozilla.org,alicoding/teach.webmaker.org,mrinaljain/teach.webmaker.org,emmairwin/teach.webmaker.org,AnthonyBobsin/teach.webmaker.org,toolness/teach.webmaker.org,alicoding/teach.mozilla.org,mozilla/teach.mozilla.org,cadecairos/teach.mozilla.org,cadecairos/teach.mozilla.org,alicoding/teach.webmaker.org,PaulaPaul/teach.mozilla.org,emmairwin/teach.webmaker.org,fredericksilva/teach.webmaker.org,fredericksilva/teach.webmaker.org,Pomax/teach.mozilla.org,PaulaPaul/teach.mozilla.org,asdofindia/teach.webmaker.org,toolness/teach.webmaker.org,ScottDowne/teach.webmaker.org,mmmavis/teach.mozilla.org,asdofindia/teach.webmaker.org,mrinaljain/teach.webmaker.org,ScottDowne/teach.webmaker.org,Pomax/teach.mozilla.org,alicoding/teach.mozilla.org,mozilla/teach.webmaker.org,mozilla/teach.mozilla.org,AnthonyBobsin/teach.webmaker.org,Pomax/teach.webmaker.org,mozilla/teach.webmaker.org,mmmavis/teach.webmaker.org,Pomax/teach.webmaker.org,mmmavis/teach.webmaker.org