Fix forder-order equivalence test for atypical collate locales

[MichaelChirico]

NB: currently targeting #7836

Running test.data.table() in an LC_ALL=lv_LV.utf8 session can error like:

...Sys.getlocale()=='...LC_COLLATE=lv_LV.utf8...'...
Error in test.data.table() : 
  387 errors out of 12808. Search tests/tests.Rraw for test numbers 1223.007, 1223.011, 1223.021, 1223.022, 1223.029, 1223.03, 1223.049, 1223.05, 1223.057, 1223.058, 1223.061, 1223.063, 1223.065, 1223.066, 1223.067, 1223.069, 1223.07, 1223.121, 1223.122, 1223.123, 1223.124, 1223.129, 1223.13, 1223.133, 1223.134, 1223.137, 1223.138, 1223.139, 1223.14, 1223.141, 1223.142, 1223.201, 1223.202, 1223.205, 1223.206, 1223.209, 1223.21, 1223.211, 1223.212, 1223.213, 1223.214, 1223.225, 1223.226, 1223.227, 1223.228, 1223.229, 1223.231, 1223.377, 1223.378, 1223.381, 1223.382, 1223.385, 1223.386, 1223.389, 1223.39, 1223.457, 1223.458, 1223.461, 1223.462, 1223.465, 1223.466, 1223.469, 1223.47, 1252.017, 1252.018, 1252.019, 1252.02, 1252.045, 1252.046, 1252.047, 1252.048, 1252.049, 1252.05, 1252.051, 1252.052, 1252.069, 1252.07, 1252.071, 1252.072, 1252.073, 1252.074, 1252.075, 1252.076, 1252.085, 1252.086, 1252.087, 1252.088, 1252.089, 1252.09, 1252.091, 1252.092, 1252.093, 1252.094, 1252.09

It's all down to xtfrm(letters) not being able to support C-collated sorting.

In 268c7d6, I tried just using order(method='radix') as much as possible, but tests 1252.* explicitly guard against doing that given it's kinda pointless to test our implementation against... whatever drift has happened between our implementations since our algorithm was first merged into {base}.

So here, I take the approach to avoid using all of letters in such cases, instead focusing on the C-sortable subset of letters.

According to Gemini, the three most adversarial locales for this exercise are lv_LV, lt_LT, and az_AZ, each of which leave only 11 letters in the chosen algorithm. I confirm that result, but it doesn't rule out there being some other more adversarial locale. Gemini also mentions haw_US and sm_WS could leave 9, but it seems like in practice those locales just use normal ASCII sorting.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.01%. Comparing base (f93fce1) to head (39d4309).

Additional details and impacted files

@@                 Coverage Diff                 @@
##           test-err-show-diff    #7837   +/-   ##
===================================================
  Coverage               99.01%   99.01%           
===================================================
  Files                      88       88           
  Lines                   17234    17234           
===================================================
  Hits                    17065    17065           
  Misses                    169      169           

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