improve unit test coverage

[andyzhangx]

Is your feature request related to a problem?/Why is this needed

Describe the solution you'd like in detail

current 61% ut coverage is too small
https://coveralls.io/github/kubernetes-sigs/sig-storage-local-static-provisioner?branch=master

Describe alternatives you've considered

Additional context

[andyzhangx]

it's now 68%

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/lifecycle stale