selftests: netfilter: nft_queue.sh: shellcheck cleanups

No functional change intended.  Disable frequent shellcheck warnings wrt.
"unreachable" code, those helpers get called indirectly from busywait helper.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240418152744.15105-3-fw@strlen.de


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