selftests: net: ksft: interrupt cleanly on KeyboardInterrupt

It's very useful to be able to interrupt the tests during development.
Detect KeyboardInterrupt, run the cleanups and exit.

Link: https://patch.msgid.link/20240705015222.675840-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>