selftests: netfilter: Avoid hanging ipvs.sh

If the client can't reach the server, the latter remains listening
forever. Kill it after 5s of waiting.

Fixes: 867d2190 ("selftests: netfilter: add ipvs test script")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>