selftests: drv-net: tso: make bkg() wait for socat to quit

Commit 846742f7e32f ("selftests: drv-net: add a warning for
bkg + shell + terminate") added a warning for bkg() used
with terminate=True. The tso test was missed as we didn't
have it running anywhere in NIPA. Add exit_wait=True, to avoid:

# Warning: combining shell and terminate is risky!
# SIGTERM may not reach the child on zsh/ksh!

getting printed twice for every variant.

Fixes: 0d0f4174f6c8 ("selftests: drv-net: add a simple TSO test")
Reviewed-by: Willem de Bruijn <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/drivers/net/hw/tso.py

···
     port = rand_port()
     listen_cmd = f"socat -{ipver} -t 2 -u TCP-LISTEN:{port},reuseport /dev/null,ignoreeof"
 
-    with bkg(listen_cmd, host=cfg.remote) as nc:
+    with bkg(listen_cmd, host=cfg.remote, exit_wait=True) as nc:
         wait_port_listen(port, host=cfg.remote)
 
         if ipver == "4":