QtTestLib update

[Daniel Teske]

> It was *never* a CS term as far as I remember. Strictly math terminology
> as mentioned below.

To clear thing up:
 
epsilon is as every greek letter, a letter with a variety of meanings. The most common in math, is that of a very small value. 

In CS epsilon is the machine error, that is every single operation a op b has an rounding error of less than epsilon. 
Based on this and on rules how different operations propagate errors, you can make estimates how different formulations of a algorithm perform in terms of rounding errors.

daniel