as.data.table.array: copy shared dimnames before reversing them

[aitap]

Fixes: #7506. Tested by installing HEAD of this PR and re-running R CMD check ctsem_*.tar.gz. It still fails tests on my machine, but due to unrelated problems in parallelly cluster startup, not in data.table. Previously, setfrev() could be called on either dimnames(x) or a list that shared names(dimnames(x)), mutating x as a result.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.04%. Comparing base (ba7a1b3) to head (036b8e1).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7509   +/-   ##
=======================================
  Coverage   99.04%   99.04%           
=======================================
  Files          87       87           
  Lines       16704    16705    +1     
=======================================
+ Hits        16544    16545    +1     
  Misses        160      160           

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[MichaelChirico]

was there not a place to just change setfrev() to frev() to get the copy?

[github-actions]

• HEAD=as.data.table-less-frev stopped early for isoweek improved in #7144
• HEAD=as.data.table-less-frev slower P<0.001 for memrecycle regression fixed in #5463
  

Generated via commit 036b8e1

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

Task	Duration
R setup and installing dependencies	5 minutes and 17 seconds
Installing different package versions	11 minutes and 47 seconds
Running and plotting the test cases	5 minutes and 8 seconds

[aitap]

Given that it took me three tries to get both tests to pass, maybe replacing setfrev with frev was a better idea.