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

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 <willemb@google.com>
Link: https://patch.msgid.link/20250604012055.891431-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>