Small changes to test formats

Okay, based upon conversations with Joanie today I made a couple of small
changes to the way output is captured and reported into the test results.
This means that the ATTA -> WPT communication is clearer and the optional
logging should be easier to interpret.

I also updated the documentation about messages and logs in the spec at [1].

[1] https://spec-ops.github.io/atta-api/index.html

-- 
Shane McCarron
Projects Manager, Spec-Ops