I just finished reading The Testing this evening (review coming tomorrow) so I decided to pick up The Testing Guide since it's listed as a prequel to the series. I'm glad that it was free, however, since it didn't actually provide any new material or answer any questions.
Set six years prior to the events in The Testing, this prequel tells the very short story of Cia's neither Zeen, and his disappointment at not being selected for testing. Though told from Zeen's POV and centered around his graduation, it's essentially a rehash of the events that will happen six years later at Cia's graduation. There are no need revelations, and minor details could have been included within two or three paragraphs maximum in The Testing.
Final opinion is that there really isn't a need for it, and you certainly won't lose out of you don't read it, but it's written well and is consistent with the story told in the first book of the series.