Playtesting is a term referring to the testing of a level to see if it is solvable.
In the MS version of the game, it appears that little to no playtesting was done, as there are numerous glitches and some changes had to be made to the levels to accommodate the differences, such as the case of Spirals corruption.
In Later Official Level Packs
In Custom Level Sets
In custom level sets, playtesting is usually done by the creator of the levelset, although it can be done by friends or family members of the creator, or other chipsters.