🧹 [Replace std::cout with using declaration in test files]

[google-labs-jules]

🎯 What: Added using std::cout; and using std::endl; to test files and removed the std:: prefix throughout.
💡 Why: This makes the code cleaner than repeatedly using the std:: prefix or pulling in the entire std namespace via using namespace std;, and resolves Linear issue GAS-5.
✅ Verification: Ran all the test executables (tests, test_framebuffer, test_vec3) locally to ensure they still build and pass without issues.
✨ Result: Cleaner, more readable test files.

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PR created automatically by Jules for task 11185547302514085661 started by @EricBorges2019

[google-labs-jules]

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