vsock/test: Add test for accept_queue memory leak

Attempt to enqueue a child after the queue was flushed, but before
SOCK_DONE flag has been set.

Test tries to produce a memory leak, kmemleak should be employed. Dealing
with a race condition, test by its very nature may lead to a false
negative.

Fixed by commit d7b0ff5a ("virtio/vsock: Fix accept_queue memory
leak").

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Link: https://patch.msgid.link/20241219-test-vsock-leaks-v4-5-a416e554d9d7@rbox.co


Signed-off-by: Jakub Kicinski <kuba@kernel.org>