tests: Reduce testsuite run-time

tests: Reduce testsuite run-time

Where acceptable, batch add set element calls to avoid overhead of
excessive 'ipset' program spawns. On my (slow) testing VM, this patch
reduces a full run of tests/runtest.sh from ~70min down to ~11min.

This might eliminate the situation being tested: resize.sh might be such
a case so batch only 255 'ipset add' calls and continue to repeat these
batched calls 32 times in hopes that it still qualifies as the resizing
stress test tests/hash:ip.t calls it.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Jozsef Kadlecsik <kadlec@netfilter.org>