test.data.table() no longer writes 'x' to .GlobalEnv

[mattdowle]

Closes #2828

[codecov-io]

Codecov Report

Merging #2830 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2830      +/-   ##
==========================================
- Coverage   93.45%   93.45%   -0.01%     
==========================================
  Files          61       61              
  Lines       12369    12367       -2     
==========================================
- Hits        11560    11558       -2     
  Misses        809      809

Impacted Files	Coverage Δ
R/test.data.table.R	100% <100%> (ø)	⬆️
src/fsort.c	73.61% <0%> (-0.32%)	⬇️

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