Improves messages generated by agent testing classes

## Context 
- Messages generated by agent testing classes were not always very clear nor verbose.
- Log files weren't ignored by git.

## Description
- Improves agent testing classes :
  * error messages are now more readable (especially when a timeout occurs);
  * actual values for `timeout`, `assertTimeout` and `maxTriesBeforeFailure` parameters are logged.
- Log files are now ignored by git.