[unit test] fix unit tests in ref_manager_test.go

[Congrool]

What would you like to be added:
We commented the tests in pkg/yurtappmanager/util/refmanager/ref_manager_test.go in #72. We have to fix these tests in further work.

This issue is created for tracing the left problem of #72. When we complete the work described above, we can close it.

Why is this needed:

others
/kind feature

[rambohe-ch]

@Congrool Thank you for adding this issue.

[River-sh]

/assign