Update test plan for next release

[Cervator]

Not that we're really doing releases yet, but I figured this would be a good habit to start: document changes that will result in needed adjustments in the test plan. Usually we'd probably do something like this later in a release cycle while reviewing the pending milestone for changes needed (unless of cause authors themselves update the test plan as they go along, which is naturally preferred)

• BlindWalk potion for GCI 2016 task #25 - adds the BlindWalk potion. Has two effects, the walk speed reduction works, but blindness is not yet implemented (possible dependency on [RFR] Introduces API for module developers to add graphical tweaks to their mods MovingBlocks/Terasology#2449)

(pinging @xrtariq2594 for reference, @nihal111 too if he ends up merging a potion or two)

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