selftests: netfilter: avoid test timeouts on debug kernels

Jakub reports that some tests fail on netdev CI when executed in a debug
kernel.

Increase test timeout to 30m, this should hopefully be enough.
Also reduce test duration where possible for "slow" machines.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240429105736.22677-1-fw@strlen.de


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