Feat/fix testcase stdout and add trace property

[b4nst]

I think #12 was actually a misread of the schema and has been merged too fast. In my understanding system-out and system-err are separated from the error or failure cause.

From the [schema] mentioned in #12:

Contains as a text node relevant data for the error, e.g., a stack trace

stack trace != stdout + stderr (even if they can be sometime related, by being a subset for example).

This PR revert the system-out and system-err, and add an additional property to error and failure types. I did not want to break the API so I introduced set_trace on TestCaseBuilder which ignore non erroneous results.

[bachp]

If I understand correctly this change introduces a proper cause for each case. Making system-err and system-out their own fields.

If this is correct and from looking at the schema this change makes sense to me.