selftests: net: py: avoid masking exceptions in bkg() failures

bkg() failures are currently quite hard to debug and spot.
Often we have code along the lines of:

  with bkg("./cmd_rx_something -p PORT"):
       wait_port_listen(PORT)
       cmd("./cmd_tx_something", host=remote)

When wait_port_listen() fails we don't get to see the exit status
of bkg(). Even tho very often it's a failure in the bkg() command
that's actually to blame. Try not to interfere with the bkg()
command error checking.

With:

   with bkg("false", exit_wait=True):
        time.sleep(0.01)  # let the 'false' cmd run
        raise Exception("bla")

Before:

  .. stack trace ..
  # Exception| Exception: bla

After:

  .. stack trace ..
  # Exception| Exception: bla
  # Exception|
  # Exception| During handling of the above exception, another exception occurred:
  .. stack trace ..
  # Exception| lib.py.utils.CmdExitFailure: Command failed: false
  # Exception| STDOUT: b''
  # Exception| STDERR: b''

Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260223202633.4126087-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>