Clean up unit tests

[jslee02]

This PR is an initial work for refactoring of the DART testing, including:

• Add dart_build_tests() to easily define tests
• Add subdirectories for unit test for each namespace (e.g., unittest/unit/common and unittest/unit/math)
• Rename comprehensive test to integration test
• Move "unit" tests that test more than one namespace to integration tests

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Before creating a pull request

• Document new methods and classes
• Format new code files using ClangFormat by running make format
• Build with -DDART_TREAT_WARNINGS_AS_ERRORS=ON and resolve build warnings

Before merging a pull request

• Set version target by selecting a milestone on the right side
• Summarize this change in CHANGELOG.md
• Add unit test(s) for this change

[codecov]

Codecov Report

Merging #1635 (ba5c42d) into main (09935e5) will increase coverage by 0.02%.
The diff coverage is 73.91%.

@@            Coverage Diff             @@
##             main    #1635      +/-   ##
==========================================
+ Coverage   58.95%   58.98%   +0.02%     
==========================================
  Files         483      485       +2     
  Lines       40252    40272      +20     
==========================================
+ Hits        23732    23754      +22     
+ Misses      16520    16518       -2     

Impacted Files	Coverage Δ
dart/common/CAllocator.cpp	71.11% <0.00%> (-0.99%)	⬇️
dart/common/detail/StlAllocator-impl.hpp	94.11% <94.11%> (ø)
dart/common/StlAllocator.hpp	100.00% <100.00%> (ø)
dart/common/MemoryAllocator.cpp	27.27% <0.00%> (+27.27%)	⬆️
dart/common/CAllocator.hpp	100.00% <0.00%> (+100.00%)	⬆️