The descriptor for test netns-nf and netns are referencing /bin/diff.
On Fedora this is a hard link to /usr/bin/diff created during the
install of the package diffutils. However this hard link doesn't exist
on Debian like systems.

Referencing /usr/bin/diff works in both cases.
Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>