LIBDRUM-897. Initial DRUM Test Plan document

Added a DRUM Test Plan document to verify as many UMD customizations
and other concerns (such as read-only containers) as feasible.

This plan is not intended to be exhaustive, instead it is designed to
provide basic assurance that the UMD customizations are in
place, and guard against regressions.

https://umd-dit.atlassian.net/browse/LIBDRUM-897