In some conditions, background processes in udpgro don't have enough
time to set up the sockets. When foreground processes start, this
results in the test failing with "./udpgso_bench_tx: sendmsg: Connection
refused". For instance, this happens from time to time on a Qualcomm
SA8540P SoC running CentOS Stream 9.

To fix this, increase the time given to background processes to
complete the startup before foreground processes start.
Signed-off-by: NAdrien Thierry <athierry@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>