Script Testing Maps are made with the sole purpose of testing out custom scripts.
They usually consist of

A example of cells for mobs.

multiple "cells" with hostile mobs inside of them. They are inside of the cells so that they can't harm you or escape but you can still test the script.
Most of these maps are either
  • on a super-flat world
  • have a source of light surrounding them 
  • or have something to prevent hostile mobs from getting in.

These maps come in various sizes can range anywhere from a small 3x3 cell with a mob inside to a huge building with every hostile mob in it. Some of these maps even go as far as having complex redstone machines to make the testing easier.
Overall one of these types of maps makes it a lot easier to test then creating a new world or building everytime you want to test a custom script.