bonnie: Expand description in config help text.

The description "Filesystem tester" is rather unhelpful. Take the
description from the referenced web site to make it a bit clearer what
bonnie++ actually is.

[Thomas: rewrap Config.in help text.]

Signed-off-by: Cam Hutchison <camh@xdna.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>