Better integrate output from slimerjs tests

A couple of changes:

-  If we're in slimer, `display_slimer_output` records effectively `not buffer_output`, and we only display the output if it would have been displayed directly from casper.
- If the return code is non-zero, e.g. because it got killed by a signal, use that rather than scanning the output.
- Booleate the match so that comparisons `ret == 0` later do the right thing (`False` is equivalent to 0).