Add an atime test for verbose=TRUE speed-up

[MichaelChirico]

          this looks like a good start for performance test, https://github.com/Anirban166/data.table/pull/13#issuecomment-2235046139 because we can clearly see the difference between Slow and HEAD but please add a Fast commit from this PR

Originally posted by @tdhock in #6296 (comment)