test: fix sending_arbitrary_iproto_packets_test hanging shutdown

test_box_iproto_send_errors test adds on disconnect callback handler
which is not cleaned up. So as it writes to channel with capacity 1 and
no one reads from it every new connection will hang on disconnect. And
we make new connection for every test in before each hook. As a result
we fail to shutdown test server in time. Iproto shutdown hangs waiting
for connections to disconnect.

Part of #8423

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