selftests: xsk: Decrease sending speed

Decrease sending speed to avoid potentially overflowing some buffers
in the skb case that leads to dropped packets we cannot control (and
thus the tests may generate false negatives). Decrease batch size and
introduce a usleep in the transmit thread to not overflow the
receiver.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210825093722.10219-12-magnus.karlsson@gmail.com