Add comment for testing in CMakeLists.txt

[zhangning21]

depends-on: [zhn-test/nuttx/pull/2 https://github.com//pull/2 https://github.com/zhn-test/nuttx/pull/3]

Note: Please adhere to Contributing Guidelines.

Summary

Update this section with information on why change is necessary,
what it exactly does and how, if new feature shows up, provide
references (dependencies, similar problems and solutions), etc.

Impact

Update this section, where applicable, on how change affects users,
build process, hardware, documentation, security, compatibility, etc.

Testing

Update this section with details on how did you verify the change,
what Host was used for build (OS, CPU, compiler, ..), what Target was
used for verification (arch, board:config, ..), etc. Providing build
and runtime logs from before and after change is highly appreciated.