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

Adrien Thierryrequested to merge
athierry1/centos-stream-9:bz-2143407-give-more-time-udpgro into main 
selftests/net: give more time to udpgro bg processes to complete startup

Bugzilla: https://bugzilla.redhat.com/2143407

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>
Edited by Adrien Thierry

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