Reset isReal test per-column to fix forder() with na.last=TRUE

[MichaelChirico]

Closes #7875

isReal must be reset on each column lest the state leak with the consequences we observe in the bug. The bug only affects combinations of real-then-non-real, but we include regression tests for real-real pairs as well (which includes real-complex).

[github-actions]

• HEAD=forder-na-last much slower for as.data.table.array improved in #7019
  

Generated via commit 87ce751

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

Task	Duration
R setup and installing dependencies	2 minutes and 52 seconds
Installing different package versions	0 seconds
Running and plotting the test cases	5 minutes and 45 seconds

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.01%. Comparing base (561a869) to head (87ce751).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7877   +/-   ##
=======================================
  Coverage   99.01%   99.01%           
=======================================
  Files          88       88           
  Lines       17292    17292           
=======================================
  Hits        17122    17122           
  Misses        170      170           

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