Avoid data.table:: for tests to work more reliably under cc()

[MichaelChirico]

If the installed version of {data.table} is still in a weird state, data.table:: here will reference it and cause some confusing issues.

Better to use the value of fread() resolved in the current session.

This is the only call site with the issue; there are other data.table:: in the file, but all are being intercepted by NSE and thus ignored:

https://github.com/Rdatatable/data.table/blob/914dd14bfddb08e53d625be21ceb0f2e65594c85/inst/tests/tests.Rraw#L18102-L18104

Merging as pretty inconsequential. cc @aitap as the author for FYI

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.01%. Comparing base (7bc8bc1) to head (4983a88).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7876   +/-   ##
=======================================
  Coverage   99.01%   99.01%           
=======================================
  Files          88       88           
  Lines       17292    17292           
=======================================
  Hits        17122    17122           
  Misses        170      170           

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