Add test comment in CMakeLists.txt

[zhangning21]

depends-on: [https://github.com/zhn-test/nuttx-apps/pull/1]

Added a comment indicating this is a test.

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

This section should provide a detailed description of what you did
to verify your changes work and do not break existing code.

Please provide information about your host machine, the board(s) you
tested your changes on, and how you tested. Logs should be included.

For example, when changing something in the core OS functions, you
may want to run the OSTest application to verify that there are no
regressions. Changes to ADC code may warrant running the adc
example. Adding a new uORB driver may require that you run
uorb_listener to verify correct operation.

Pure documentation changes can just be tested with make html
(see docs) and verification of the correct format in your
browser.

PRs without testing information will not be accepted. We will
request test logs.

[zhangning21]

Here's a working version running in my own forks (zhn-test/nuttx + zhn-test/nuttx-apps), with CI logs for each test case.

Note: on the fork the dependent PR is fetched from the fork's origin, so the examples use zhn-test/... instead of apache/...; the behavior is identical. The relevant
evidence is the Apply depends-on PRs step in the Fetch-Source job.

#	Test case	depends-on value	Apply step	Result	Log
1	No depends-on	(none)	skipped	Baseline; CI proceeds	run
2	Valid PR number	[zhn-test/nuttx-apps/pull/1]	success	Fetched + cherry-picked	run
3	Invalid PR number	[zhn-test/nuttx/pull/88888888]	failure	fatal: couldn't find remote ref → exit 1 (fail-fast)	run
4	Non-Apache repo	[lupyuen/nuttx/pull/1]	skipped	Ignored by allowlist; CI proceeds	run
5	Typo (push/)	[zhn-test/nuttx/push/2]	skipped	Not matched; CI proceeds	run

Note: cases 4 and 5 are silently ignored (no error, no warning) because the values don't match the dependency regex. If preferred, I can add a warning when a depends-on: line is
present but yields no valid dependency, to catch typos/wrong repos early.