Avoid using structure() in tests

[MichaelChirico]

Follow-up to #7813.

structure() is slow, and, as we saw there, can be a bit fragile. It's better to use the greenlit path to construct objects where possible.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.01%. Comparing base (f55ed35) to head (8102bf2).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7814   +/-   ##
=======================================
  Coverage   99.01%   99.01%           
=======================================
  Files          88       88           
  Lines       17228    17234    +6     
=======================================
+ Hits        17059    17065    +6     
  Misses        169      169           

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[github-actions]

• HEAD=structure much slower for as.data.table.array improved in #7019
• HEAD=structure much slower for transform improved in #5493
• HEAD=structure slower P<0.001 for memrecycle regression fixed in #5463
  

Generated via commit 8102bf2

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

Task	Duration
R setup and installing dependencies	6 minutes and 49 seconds
Installing different package versions	12 minutes and 7 seconds
Running and plotting the test cases	5 minutes and 31 seconds