Use setattr to avoid shallow copy in autoprint test setup

[MichaelChirico]

Part of #7099

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.69%. Comparing base (b19acf1) to head (7425fb4).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7102   +/-   ##
=======================================
  Coverage   98.69%   98.69%           
=======================================
  Files          79       79           
  Lines       14680    14680           
=======================================
  Hits        14489    14489           
  Misses        191      191           

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[jangorecki]

Two of dependencies could not be installed in this job

Warning message:
packages ‘xts’, ‘knitr’ are not available (for R version 3.4.0) 

warning is gone but problem with matching output is still there, maybe for 3.4.0 we need different mockup file.
https://gitlab.com/Rdatatable/data.table/-/jobs/10484872441

https://gitlab.com/Rdatatable/data.table/-/jobs/10484872441/artifacts/browse/bus/test-lin-ancient-cran/data.table.Rcheck/tests/

[jangorecki]

Looks like

DT[2,a:=3L]

(amongst others) prints in 3.4.0

[MichaelChirico]

Thanks. Looks like this is an improvement, but doesn't fix the issue. I think worth merging anyway.

[jangorecki]

yes, worth merging. Assuming R 3.5.0 is not being affected I think we could ignore the printing issue for R 3.4.0. At least till nobody reported that till now.