Unit tests: Remove unnecessary make_unique calls

[ajor]

This was a purely mechanical change made with some find-and-replace macros. The tests still pass afterwards.

The second commit I'm bundling into this PR is a lot smaller and just moves the last remaining tests onto using the type_graph_utils helpers.

[codecov-commenter]

Codecov Report

Merging #205 (8ed7fa7) into main (01c9573) will decrease coverage by 0.05%.
The diff coverage is 97.35%.

@@            Coverage Diff             @@
##             main     #205      +/-   ##
==========================================
- Coverage   65.01%   64.97%   -0.05%     
==========================================
  Files          87       87              
  Lines        9487     9478       -9     
  Branches     1568     1568              
==========================================
- Hits         6168     6158      -10     
  Misses       2486     2486              
- Partials      833      834       +1     

Impacted Files	Coverage Δ
test/test_add_padding.cpp	91.66% <88.88%> (ø)
test/test_flattener.cpp	95.03% <95.45%> (ø)
test/test_alignment_calc.cpp	97.29% <96.55%> (-0.21%)	⬇️
test/test_name_gen.cpp	99.15% <100.00%> (-0.02%)	⬇️
test/test_remove_top_level_pointer.cpp	100.00% <100.00%> (ø)
test/test_topo_sorter.cpp	99.14% <100.00%> (-0.06%)	⬇️

... and 2 files with indirect coverage changes

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