Tests - Update syntax and improve implementation

Update syntax, style, documentation and contracts for the testing suite.

Replace Assigment attempt with object test.
USe across loops instead of traditional loops.
Use a better implementation for converters.
I noted a mix between space and tabulations on each files and sometimes on the same line.
This mix was already existing before my commits.

Git reports the mix when it is on the same line. Then, for each new line introduced, I decided to use tabs.
Since tabs and space have the same length on Eiffel Studio, the printing is ok, however on Github the length is not the same...

Maybe it could be better to choose between space or tabs and then to make a global and evil update...