Flaky test: TestFdLeaks

[kolyshkin]

Description

From https://github.com/opencontainers/runc/actions/runs/9307874551/job/25620077978?pr=4290 (job is ubuntu-22.04, 1.21.x, -race):

=== RUN   TestFdLeaks
    exec_test.go:1759: extra fd 23 -> 
    exec_test.go:1762: found 1 extra fds after container.Run
--- FAIL: TestFdLeaks (0.22s)

This is the first time I see it.

[YanzhaoLi]

1. runContainer
2. fd0 := /proc/thread-self/fd/
3. runContainer
4. fd1 := /proc/thread-self/fd/ // Which should be the fds of the calling process

I wonder if the TestFdLeaks is checking the leakage of FDs that was created by Runc, instead of the FDs created by container process