selftests: drv-net: psp: fix test flakes from racy connection close

There is a bug in assoc_sk_only_mismatch() and
assoc_sk_only_mismatch_tx() that creates a race condition which
triggers test flakes in later test cases e.g. data_send_bad_key().

The problem is that the client uses the "conn clr" rpc to setup a data
connection with psp_responder, but never uses a matching "data close"
rpc. This creates a race condition where if the client can queue
another data sock request, like in data_send_bad_key(), before the
server can accept the old connection from the backlog we end up in a
situation where we have two connections in the backlog: one for the
closed connection we have received a FIN for, and one for the new PSP
connection which is expecting to do key exchange.

From there the server pops the closed connection from the backlog, but
the data_send_bad_key() test case in psp.py hangs waiting to perform
key exchange.

The fix is to properly use _conn_close, which fill force the server to
remove the closed connection from the backlog before sending the RPC
ack to the client.

Signed-off-by: Daniel Zahka <daniel.zahka@gmail.com>
Link: https://patch.msgid.link/20260127-psp-flaky-test-v1-1-13403e390af3@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>