Add TestIP that uses cpu on 4 Linux architectures

[rminnich]

The test runs on Linux on amd64, arm, arm64, and riscv64.

Once this is merged, this test can replace the one in u-root. Our goal is to elminate as many u-root VM shell script tests as possible, because we have found undetected errors in them. Tests should be written in Go.

We keep this test here to also test cpu. Getting it all working was no small matter. We want it to keep working :-)

We extend the client package in small ways to make writing VM tests convenient.

The kernel images and initramfs for cpu are built into the package. The tests can be run hermetically by running go test.
ghcr.io is only used for go generate, and that will not be frequent (possibly changing only every year or so).

The kernels are cached from the u-root ghcr.io docker images. The kernels have a few tweaks, for things missing
from the kernel (GRE, VTI) in ghcr.io. The PR for these additions has been submitted for the vmtest repo. The kernels also provide /proc/config.gz. For now, my fixes are local to config_linux.txt files.

Note: I have no idea how the vmscript in u-root is passing at this point, with support for GRE, VTI, and other things missing. The script may not be working; there may be errors that are not caught. I've seen ignored errors in other scripts.

The compiled-in initramfs package only contains cpud. The vm package builds a u-root image for testing as needed and concatenates it to the cpud initramfs.

The VMs start with two network interfaces. The first is used to support cpud; the second interface can be manipulated by the tests. This removes the need to parse VM console output, which can be fragile.

IP is configured by ip=dhcp.

The test assumes there is a working qemu on the CI. This allows us to run these tests on any kernel/architecture combination that has qemu.

[Copilot]

Pull Request Overview

This PR adds comprehensive integration tests for the cpu package using QEMU VMs across four Linux architectures (amd64, arm, arm64, and riscv64). The tests validate network configuration capabilities using the u-root ip command, including interface management, routing, tunneling (SIT, GRE, VTI, IPIP), and ARP operations. This replaces fragile shell-based VM tests in u-root with hermetic Go tests that can be run with go test.

Key Changes

• Added TestIP function that validates ip command functionality across multiple architectures
• Extended vm package API to support architecture-specific builds with GOOS/GOARCH parameters
• Added second network interface to VMs for testing network operations without parsing console output
• Updated TestCPUAMD64 to use new Uroot API and added similar TestCPUARM test

Reviewed Changes

Copilot reviewed 4 out of 14 changed files in this pull request and generated 9 comments.

File	Description
vm/ip_linux_test.go	New comprehensive test validating ip command operations across four architectures
vm/vm.go	Extended Image struct and Uroot function to support architecture-specific builds; added second network interface
vm/vm_linux_test.go	Updated existing tests to use new Uroot API signature and added ARM-specific test
vm/build.go	Reordered build configurations and added debug code (disabled) for container image pulling

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[brho]

minor nits

[brho]

do you want both the cpio and the cpio.gz?

[rminnich]

Sorry I missed your comments. I will work on them next.