selftests: net: py: check process exit code in bkg() and background cmd()

We're a bit too loose with error checking for background
processes. cmd() completely ignores the fail argument
passed to the constructor if background is True.
Default to checking for errors if process is not terminated
explicitly. Caller can override with True / False.

For bkg() the processing step is called magically by __exit__
so record the value passed in the constructor.

Reported-by: Willem de Bruijn <willemb@google.com>
Tested-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20240502025325.1924923-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>