test/vsock: Add ioctl SIOCINQ tests

Add SIOCINQ ioctl tests for both SOCK_STREAM and SOCK_SEQPACKET.

The client waits for the server to send data, and checks if the SIOCINQ
ioctl value matches the data size. After consuming the data, the client
checks if the SIOCINQ value is 0.

Signed-off-by: Xuewei Niu <niuxuewei.nxw@antgroup.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Tested-by: Luigi Leonardi <leonardi@redhat.com>
Reviewed-by: Luigi Leonardi <leonardi@redhat.com>
Link: https://patch.msgid.link/20250708-siocinq-v6-4-3775f9a9e359@antgroup.com


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