Update nsupdate test

Update nsupdate test

The nsupdate system test did not record failures from the
'update_test.pl' Perl script. This was because the 'ret' value was
not being saved outside the '{ $PERL ... || ret=1 } cat_i' scope.

Change this piece to store the output in a separate file and then
cat its contents. Now the 'ret' value is being saved.

Also record failures in 'update_test.pl' if sending the update
failed.

Add missing 'n' incrementals to 'nsupdate/test.sh' to keep track of
test numbers.

(cherry picked from commit 5b31811b5f15c7677a3f1a65b8dcc23d4a55f3d4)