Update locale in test.data.table

[ColeMiller1]

On Windows, the locale would not get reset after the test.data.table() was complete. For me, that meant that the first test would be successful but then I had these weird UTC errors.

The issue was caused by on.exit() being used in a for loop for a test. It turns out when you use a variable assigned in a for loop within on.exit(), the last instance of the variable is used on exit. Follow-up to #4524 .

double_on_exit = function() {
    for (i in 1:2) {
        lc = letters[i]
        on.exit(print(lc), add = TRUE)
    }
    on.exit(print('bye'), add = TRUE)
}
double_on_exit()
#> [1] "b"
#> [1] "b"
#> [1] "bye"

[codecov]

Codecov Report

Merging #4630 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4630   +/-   ##
=======================================
  Coverage   99.61%   99.61%           
=======================================
  Files          73       73           
  Lines       14244    14244           
=======================================
  Hits        14189    14189           
  Misses         55       55           

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