Add unit testing for pipeline test runner to cover json decoding and encoding

[jsoriano]

In #717 a json decoder was introduced to parse numbers as json numbers instead of float64. This is needed to avoid truncating big integers, what can contaminate test files and can hide actual problems.

Add unit testing in a way that allows to check that the final written result is exactly what is expected, covering all the uses of json decoding and encoding.

Some ideas can be found in this comment #717 (comment) and in #721.