selftests: net: udpgso_bench: Fix racing bug between the rx/tx programs

selftests: net: udpgso_bench: Fix racing bug between the rx/tx programs

"udpgro_bench.sh" invokes udpgso_bench_rx/udpgso_bench_tx programs
subsequently and while doing so, there is a chance that the rx one is not
ready to accept socket connections. This racing bug could fail the test
with at least one of the following:

./udpgso_bench_tx: connect: Connection refused
./udpgso_bench_tx: sendmsg: Connection refused
./udpgso_bench_tx: write: Connection refused

This change addresses this by making udpgro_bench.sh wait for the rx
program to be ready before firing off the tx one - up to a 10s timeout.

Fixes: 3a687bef148d ("selftests: udp gso benchmark")
Signed-off-by: Andrei Gherzan <andrei.gherzan@canonical.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Willem de Bruijn <willemb@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20230201001612.515730-3-andrei.gherzan@canonical.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>