selftests: netfilter: nft_concat_range.sh: reduce debug kernel run time

Even a 1h timeout isn't enough for nft_concat_range.sh to complete on
debug kernels.

Reduce test complexity and only match on single entry if
KSFT_MACHINE_SLOW is set.

To spot 'slow' tests, print the subtest duration (in seconds) in
addition to the status.

Add new nft_concat_range_perf.sh script, not executed via kselftest,
to run the performance (pps match rate) tests.

Those need about 25m to complete which seems too much to run this
via 'make run_tests'.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240430145810.23447-1-fw@strlen.de


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