[unit test] improve unit test coverage for yurt-device-controller

[wawlian]

What would you like to be added:
improve unit test coverage for yurt-device-controller:

target coverage: 70%

Why is this needed:
improve unit test coverage for OpenYurt project.

others
/kind feature

[openyurt-bot]

@wawlian: The label(s) kind/feature cannot be applied, because the repository doesn't have them

In response to this:

What would you like to be added:
improve unit test coverage for yurt-device-controller:

target coverage: 70%

Why is this needed:
improve unit test coverage for OpenYurt project.

others
/kind feature

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[gnunu]

/assign @LindaYu17