selftests: netfilter: xt_string.sh: shellcheck cleanups

no functional change intended.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240418152744.15105-7-fw@strlen.de


Signed-off-by: Jakub Kicinski <kuba@kernel.org>