selftests: net: py: add cmd info for ksft_wait failure

Gal recently complained:

  When [ksft_wait failure] happens, the test fails with a cryptic
  message:
    # Exception| Exception: Did not receive ready message

Let's try to include the stdout/stderr of the command we tried
to start. E.g. for cmd("false", ksft_wait=True):

    # Exception| lib.py.utils.CmdInitFailure: Did not receive ready message
    # Exception| CMD: false
    # Exception|   EXIT: 1

We need to factor out _process_terminate() otherwise the exit
path may try to write to already disconnected self.ksft_term_fd.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260223202633.4126087-4-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>