example/libtest: add malloc/free to libtest

[XuNeo]

Summary

So we can verify flags like kasan are effective or not.

Impact

Minor

Testing

Compile libtest and disassemble library_test to verify kasan is actually used.

[nuttxpr]

[Experimental Bot, please feedback here]

No, this PR does not meet the NuttX requirements.

Missing Information:

• Summary:
  • The summary is too vague. It doesn't explain:
    • Why is the change necessary? What problem does it solve?
    • What functional part of the code is being changed? Be specific.
    • How does the change work? Provide technical details.
  • Include any relevant issue references.
• Impact:
  • Minor is not descriptive enough.
  • Address all impact points with specific details (even if the answer is NO).
• Testing:
  • Insufficient detail:
    • Specify the exact build hosts (OS, CPU, compiler versions) and targets (arch, board, config) used.
    • Provide complete and meaningful testing logs. "Compile libtest" and "disassemble library_test" are not informative enough.

Recommendations:

1. Expand the Summary: Provide a clear and detailed explanation of the problem, the solution, and how it works.
2. Address All Impact Points: Go through each impact category and provide a specific answer, even if it's "NO." Explain any "YES" answers thoroughly.
3. Provide Detailed Testing Information: List all build hosts and targets used, and include complete and relevant testing logs that demonstrate the change's functionality.