selftests: netfilter: simplify command testing

Fix some shellcheck SC2181 warnings:
"Check exit code directly with e.g. 'if mycmd;', not indirectly with
$?." as suggested by Stefano Brivio.

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>