测试：345 个集成测试因内核缺失而失败 / 345 integration tests fail due to missing kernel

## 问题描述 / Problem

Running `swift test` results in 345 test failures, all caused by `.binaryNotFound` error. These failures are not code defects — they occur because the test environment lacks the required Linux kernel installation.

运行 `swift test` 会导致 345 个测试失败，均由 `.binaryNotFound` 错误引起。这些失败不是代码缺陷，而是测试环境缺少所需的 Linux 内核安装。

## 根本原因 / Root Cause

The `CLITest` base class (`Tests/CLITests/Utilities/CLITest.swift`) requires:
1. The `container` CLI binary at `bin/container`
2. A Linux kernel installed via `container system kernel set --recommended`
3. The container system service running (`container system start`)

Most integration tests (kernel set, container lifecycle, build, network, volumes, etc.) depend on these prerequisites.

## 受影响的测试 / Affected Tests

All 345 failures span ~15 test suites including:
- `TestCLIKernelSet` — directly tests kernel operations
- `TestCLIRunCommand1/2/3`, `TestCLIRunBase`, `TestCLIRunLifecycle` — container run
- `TestCLIExecCommand`, `TestCLICreateCommand` — container management
- `TestCLIBuildBase` — image building
- `TestCLIImagesCommand`, `TestCLIRegistry` — image management
- `TestCLIPruneCommand`, `TestCLIStatsCommand` — container operations
- `TestCLIVolumes`, `TestCLIAnonymousVolumes` — volume management
- `TestCLINetwork`, `TestCLIMachineCommand`, `TestCLIMachineRuntime` — networks and machines

## 修复方案 / Fix

PR #1728 provides:
- `scripts/setup-test-env.sh` — automated test environment setup
- `docs/testing.md` — comprehensive testing guide
- Makefile improvements — `setup-test-env` and `integration-test` targets

## 临时解决方法 / Workaround

```bash
# Manual setup:
make cli
container system start # Answer "y" when prompted for kernel install
swift test
```

Start with Tests/CLITests/Utilities/CLITest.swift and review the prerequisites for the affected integration suites. Then inspect scripts/setup-test-env.sh, docs/testing.md, and the Makefile changes described in PR #1728. Done means the test environment can be set up reproducibly and swift test no longer produces these environment-caused failures.

Written by the indexing model from the issue text.