vsock/test: Add test for an unexpectedly lingering close()

There was an issue with SO_LINGER: instead of blocking until all queued
messages for the socket have been successfully sent (or the linger timeout
has been reached), close() would block until packets were handled by the
peer.

Add a test to alert on close() lingering when it should not.

Signed-off-by: Michal Luczaj <mhal@rbox.co>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://patch.msgid.link/20250522-vsock-linger-v6-5-2ad00b0e447e@rbox.co


Signed-off-by: Paolo Abeni <pabeni@redhat.com>