nsexec logging is racy (aka "debug.bats is flaky")

[kolyshkin]

Sometimes debug.bats test cases are failing like this:

not ok 27 global --debug to --log --log-format 'json'
# (in test file tests/integration/debug.bats, line 77)
#   `[[ "${output}" == *"child process in init()"* ]]' failed

This is caused by the fact that log forwarding from nsexec is not waiting for all logs to be written to disk ( before runc process ends, and in case of short-lived processes this result in missing debug from nsexec.

This happens when writing to disk, and may probably be made 100% reproducible by slowing down disk I/O.

This is probably not an issue for any real customers (no one except developers is interested in nsexec debug log), but it's at least an issue with the test (thus the subject).

I tried to fix it in #2487 (last commit) but it needs more work.