Flaky TestHook

[kolyshkin]

Seeing this for the second time:

=== RUN   TestHook
time="2026-04-13T17:48:47Z" level=warning msg="cannot serialize hook of type configs.FuncHook, skipping"
.....
time="2026-04-13T17:48:47Z" level=warning msg="cannot serialize hook of type configs.FuncHook, skipping"
    exec_test.go:1090: unexpected error: exit status 1
--- FAIL: TestHook (0.11s)

1. https://github.com/opencontainers/runc/actions/runs/24150883471/job/70477231257?pr=5222 (from tests/int/checkpoint: drop unneeded tests #5222).
2. https://github.com/opencontainers/runc/actions/runs/24358098981/job/71130299998 (from [debug/DNM] test from pull 5242 #5244).

Once I saw it I opened #5232 to be able to find out what's going on, but it is still not merged.

[kolyshkin]

OK, with the help of #5232 we now know more:

=== RUN   TestHook
time="2026-04-18T02:22:51Z" level=warning msg="cannot serialize hook of type configs.FuncHook, skipping"
...
time="2026-04-18T02:22:51Z" level=warning msg="cannot serialize hook of type configs.FuncHook, skipping"
    exec_test.go:1098: command failed: exit status 1; stderr:
        ls: /poststart: No such file or directory
--- FAIL: TestHook (0.07s)

This is related to #5186. The test case runs "ls -l .... /poststart" as a container init, and now poststart hook is run after container init is started, so the poststart hook is racing with container init.

I don't know a good way to handle it; an OK way is to add a sleep so the poststart hook wins the race.