add functional tests for cgroup v2 in gcs

[jiechen0826]

Summary

Adds functional end-to-end tests for cgroup operations in the GCS, covering both v1 and v2 hierarchies. These tests run inside the UVM as part of the GCS test binary and exercise the full cgroup stats pipeline regardless of which cgroup version the kernel provides.

Motivation

The GCS cgroup code path had no dedicated functional test coverage. As we migrate UVM images from cgroup v1 to v2 (e.g., Ubuntu 24.04), we need a regression gate that validates stats collection, resource limits, CPU throttling, and OOM monitoring work correctly on both cgroup versions.

Changes

New: test/gcs/cgroup_test.go (7 tests)

Test	What it validates
TestCgroupVersionDetection	IsCgroupV2() matches actual /sys/fs/cgroup filesystem layout
TestContainerStats_CgroupMetrics	Full stats pipeline returns non-nil memory, CPU, and pids metrics
TestContainerStats_MemoryLimit	OCI memory limit (128 MiB) round-trips correctly through cgroup
TestContainerExec_CgroupIsolation	Exec'd process lands in the correct /containers/<id> cgroup path
TestMultipleContainers_IndependentStats	Two containers with different memory limits (64 MiB / 128 MiB) report independent stats
TestContainerStats_CPUThrottling	CFS quota (10%) produces non-zero throttling counters under CPU burn
TestOOMEventFD_ContainerKill	64 MiB container killed by 128 MiB allocation reports OomKill=1 and fires eventfd

Modified: internal/guest/cgroup/cgroup.go

• Added LoadManager(cgroupPath string) (Manager, error) — loads an existing cgroup (created by the runtime) as a Manager, supporting both v1 and v2. Used by tests to inspect container cgroups.

Modified: test/gcs/main_test.go

• Fixed setup() to handle both cgroup v1 and v2 when joining the root cgroup. Previously only used the v1 API (cgroups.Load), which fails on v2-only kernels.

Testing

All 7 tests pass in a Hyper-V LCOW UVM with cgroup v2 (Ubuntu 24.04 rootfs):
--- PASS: TestCgroupVersionDetection (0.00s)
--- PASS: TestContainerStats_CgroupMetrics (2.07s)
--- PASS: TestContainerStats_MemoryLimit (2.09s)
--- PASS: TestContainerExec_CgroupIsolation (2.17s)
--- PASS: TestMultipleContainers_IndependentStats (4.17s)
--- PASS: TestContainerStats_CPUThrottling (4.15s)
--- PASS: TestOOMEventFD_ContainerKill (2.83s)
PASS