Replace Front/Back-end servlets with single TestServlet (#2874)

The servlets, at this point now that we're off GAE, are only used for
the test server (and, indirectly, in one BSA test). Instead of having
them all remain separate, we can unify them in one test servlet that
lives in the test/ folder.

This removes one avenue of potential confusion w/r/t how request routing
actually works and where we would want to add new routing.

Author: gbrodman