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

It's now 69%, target num: 85%

https://coveralls.io/github/kubernetes-sigs/blob-csi-driver?branch=master

Describe alternatives you've considered

Additional context

[mukhoakash]

/assign

[aarongalang]

/assign

[mukhoakash]

/unassign

[ganesan23]

/assign