selftests/net: give more time to udpgro bg processes to complete startup

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: Adrien Thierry <athierry@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>