selftests: netfilter: nft_flowtable.sh: re-run with random mtu sizes

Now that the test runs much faster, also re-run it with random MTU sizes
for the different link legs.  flowtable should pass ip fragments, if
any, up to the normal forwarding path.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240423130604.7013-5-fw@strlen.de


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