test/cp: fix test_copy_through_dangling_symlink_no_dereference_permissions

[niyaznigmatullin]

The test checked -p option that shouldn't change timestamps.

According to coreutils documentation, it shouldn't change atime and mtime, but changes ctime.

     ‘timestamps’
          Preserve the times of last access and last modification, when
          possible.

...

     Using ‘--preserve’ with no ATTRIBUTE_LIST is equivalent to
     ‘--preserve=mode,ownership,timestamps’.

GNU coreutils cp works like it's written in documentation in this case.

The test was passing most of the time, because it was checking the timestamp in seconds, and in a single second both the original link and copied link were created, so their timestamps were equal. Sometimes it was failing, for instance here https:/uutils/coreutils/runs/7711138977?check_suite_focus=true.

• added 5 seconds timeout between file creation and copy, so that timestamps are for sure different (after adding this it started failing every time because of ctime has been changed by 5)
• removed ctime checking
• made atime and mtime checking more accurate, started checking atime_nsec and mtime_nsec as well

[sylvestre]

I was concerned that we would slow down a lot the test execution with the sleep(5s) but it seems that it is already the case in other places. Looking at:
cargo test --features=unix -- --test-threads 1