Unlike the other map tests which heavily rely on mocks, this one uses
React refs to inspect the constructed leaflet objects. The DOM itself
doesn't appear to contain anything worth testing against (react-leaflet
rendering doesn't work like typical React rendering).
This required some infrastructure changes:
1. Forwarded ref from WaypointMarker to Marker so the test can observe
it. Added a mergeRefs helper (and its own tests) to make that easier.
2. Switched from identity-obj-proxy to jest-transform-stub, because the
former doesn't produce a useable image for imports, and we need
usable images for leaflet to be able to render.
This doesn't yet test drag and drop behavior since that requires mocking
the backend, and this commit is already complicated enough. That'll be
next.
