selftests: xsk: add timeout to tests

Add a timeout to the tests so that if all packets have not been
received within 3 seconds, fail the ongoing test. Hinders a test from
dead-locking if there is something wrong.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/r/20220510115604.8717-6-magnus.karlsson@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>