Support drop=TRUE in subset.data.table

[MichaelChirico]

Closes #7859

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.01%. Comparing base (0d338a9) to head (42e4bd7).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7860   +/-   ##
=======================================
  Coverage   99.01%   99.01%           
=======================================
  Files          88       88           
  Lines       17291    17292    +1     
=======================================
+ Hits        17121    17122    +1     
  Misses        170      170           

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[github-actions]

• HEAD=subset-drop much slower for transform improved in #5493
• HEAD=subset-drop slower P<0.001 for DT[by,verbose=TRUE] improved in #6296
  

Generated via commit 42e4bd7

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

Task	Duration
R setup and installing dependencies	3 minutes and 12 seconds
Installing different package versions	47 seconds
Running and plotting the test cases	5 minutes and 41 seconds

[ben-schwen]

change LGTM. some minor test things

[ben-schwen]