zdtm: Use /usr/bin/diff instead of /bin/diff

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>

test/zdtm/live/static/netns-nf.desc

-{'flavor': 'ns uns', 'deps': [ '/bin/sh', '/sbin/iptables', '/usr/lib64/xtables/libxt_standard.so', '/bin/diff' ], 'flags': 'suid'}
+{'flavor': 'ns uns', 'deps': [ '/bin/sh', '/sbin/iptables', '/usr/lib64/xtables/libxt_standard.so', '/usr/bin/diff' ], 'flags': 'suid'}

test/zdtm/live/static/netns.desc

-{'flavor': 'ns uns', 'deps': [ '/bin/sh', '/sbin/ip', '/bin/diff'], 'flags': 'suid'}
+{'flavor': 'ns uns', 'deps': [ '/bin/sh', '/sbin/ip', '/usr/bin/diff'], 'flags': 'suid'}