fix rounding warning in atime test for [.data.table

[tdhock]

I was getting an off by one warning due to rounding, which below is converted to error with options(warn=2)

> N <- 5623
> d <- data.table(
+ id = sample(c(seq.int(N * 0.9), sample(N * 0.9, N * 0.1, TRUE))),
+ v1 = sample(5L, N, TRUE),
+ v2 = sample(5L, N, TRUE)
+ )
Erreur dans as.data.table.list(x, keep.rownames = keep.rownames, check.names = check.names,  : 
  (converti depuis l'avis) L'élément 1 a 5622 lignes mais l'élément le plus long en a 5623 ; Il est recyclé avec le reste.

This PR fixes that warning.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.69%. Comparing base (8647d44) to head (07f54a6).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7024   +/-   ##
=======================================
  Coverage   98.69%   98.69%           
=======================================
  Files          79       79           
  Lines       14677    14678    +1     
=======================================
+ Hits        14486    14487    +1     
  Misses        191      191           

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[github-actions]

Generated via commit 07f54a6

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	4 minutes and 21 seconds
Installing different package versions	8 minutes and 14 seconds
Running and plotting the test cases	1 minutes and 57 seconds