selftests: rds: Stop tcpdump on timeout

selftests: rds: Stop tcpdump on timeout

The timeout signal handler for the rds selftests currently just
exits when the time limit is exceeded, and forgets to stop the
network dumps.  Fix this by hoisting the tcpdump terminate commands
into a helper function, and call it from the signal handler before
exiting

Bound proc.wait() with a timeout (and fall back to proc.kill())
so an unresponsive tcpdump cannot hang the timeout path itself.

We also pop() tcpdump_procs as we iterate, so stop_pcaps() is safe
to call from both the normal cleanup path and the signal handler,
since the second invocation simply has nothing to do

Signed-off-by: Allison Henderson <achender@kernel.org>
Link: https://patch.msgid.link/20260504054143.4027538-9-achender@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>