cpp: test: Only test multi_array assertions for release builds

Strict bounds checks are only compiled in when `NDEBUG` is not defined (i.e. is a non-debugging release build with assertions disabled).

/cc @joshmoore 

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Testing:

- Run cmake with `-DCMAKE_BUILD_TYPE=Release`
- The pixelbuffer tests should now pass with this change merged
- This won't be tested by our current CI setup; all builds are currently Debug builds