selftests: net: py: support ksft ready without wait

selftests: net: py: support ksft ready without wait

There's a common synchronization problem when a script (Python test)
uses a C program to set up some state (usually start a receiving
process for traffic). The script needs to know when the process
has fully initialized. The inverse of the problem exists for shutting
the process down - we need a reliable way to tell the process to exit.

We added helpers to do this safely in
commit 71477137994f ("selftests: drv-net: add a way to wait for a local process")
unfortunately the two operations (wait for init, and shutdown) are
controlled by a single parameter (ksft_wait). Add support for using
ksft_ready without using the second fd for exit.

This is useful for programs which wait for a specific number of packets
to rx so exit_wait is a good match, but we still need to wait for init.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20251120021024.2944527-7-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>