Each patchable server variant will have to download the appropriate mongot-localdev binary (as there are three diff OS binaries) and place it in the correct path for resmoke to spin up in our tests.

This ticket allows mongot engineers to run unmerged changes
to 10gen/mongot against tests defined in 10gen/mongod. This is made
possible by the server downloading the upstream patch's mongot binaries
from urls passed via downstream_expansions.