Tests used to silently pass when vcddiff aborted. Now fixed. Updated
large array trace reference files for FST, added same reference files
for VCD.
Developers need to update their local vcddiff.
The FST trace timescale used to be set in the constructor via
set_time_unit, but at that point we haven't normally opened the
file yet so it was just dropped. On top of that, we actually want
to use set_time_resolution... FST trace timescales now match the VCD.
