os: fix TestLFNIndex test warnings

Do not ignore system return value. Wrap emacs compile line to 80
characters.

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #617 from dachary/wip-lfnindex

os: fix TestLFNIndex test warnings

Reviewed-by: Sage Weil <sage@inktank.com>