MongoDB URL downloader for testing needs updates to 'latest'

The affected variants are mac ("osx" became "macos") and windows ("2008plus" became "2012plus"), on the 4.0 and master branches. If you are using the old spellings, then you're actually getting 4.1.0 for osx latest, and 3.5.5 for windows latest. The new spellings give version 4.1.7.