Flaky test: TestCompletion (` Expected stdout to contain "native\n"`)

[AkihiroSuda]

=== RUN   TestCompletion
    completion_linux_test.go:31: assertion failed: 
        Command:  /usr/local/bin/nerdctl --namespace=nerdctl-test __complete --snapshotter 
        ExitCode: 0
        Stdout:   :1
        Stderr:   Completion ended with directive: ShellCompDirectiveError
        Failures:
        Expected stdout to contain "native\n"
--- FAIL: TestCompletion (0.08s)

cgroup2 rootless on Cirrus CI, commit de19a6338d2a0a71f2f292ae3e6605e91794f753

Looks like stdio streams aren't correctly flushed