data.table convert old benchmark PR to atime

[tdhock]

Hi @DorisAmoakohene @Anirban166 can you please check https://github.com/Rdatatable/data.table/pull/4517/files and figure out if we can convert any of that code to atime performance tests for data.table? If I understand correctly, that code is for comparing a given size N (not varying N), to a given reference timing (in seconds?), so I'm not sure if they can be translated to atime.