selftests: rds: Register network teardown via atexit

This patch adds a teardown_tcp() helper that removes net0/net1.
The cmd calls here use fail=False so they can be called from
completed or partially-setup states on error. Also call
teardown_tcp() at the top of setup_tcp() so a previous
interrupted run does not leave net0/net1 lingering and break a
subsequent ip netns add.  Register teardown_tcp() with atexit
before setup_tcp() is invoked.

Likewise, we can simpliy stop_pcaps() handling by registering it
with atexit instead of calling it from the signal handler.

atexit handlers run on any exit path - normal completion, raised
exception, and sys.exit() from the timeout signal handler.  This
guarantees cleanup are called without further wrapping the test
body in a try/finally blocks.

atexit LIFO ordering keeps stop_pcaps before teardown_tcp so
tcpdumps are killed cleanly before their namespaces go away.

This is a preparatory cleanup for the upcoming ROCE patch which
will also register a teardown_rdma() alongside teardown_tcp()

Signed-off-by: Allison Henderson <achender@kernel.org>
Link: https://patch.msgid.link/20260518012443.2629206-10-achender@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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