Output the diffs to standard error when comparing the expected vs the
obtained output from lastcomm instead of just printing out a summary, e.g.
"they differed".

This will make failures with results more apparent when running kyua debug,
kyua report-html, etc.

Differential Revision: https://reviews.freebsd.org/D7079 (as part of a larger diff)
MFC after: 1 week
Approved by: re (gjb)
Reviewed by: cem
Sponsored by: EMC / Isilon Storage Division