Add UT cases to pkg/cubefs

[kaichiachen]

There are no unit test cases in pkg/cubefs so I add some - from 0% to 21.7% coverage

#> go test -cover ./...

        github.com/cubefs/cubefs-csi/cmd                coverage: 0.0% of statements
ok      github.com/cubefs/cubefs-csi/pkg/csi-common     (cached)        coverage: 28.4% of statements
ok      github.com/cubefs/cubefs-csi/pkg/cubefs 2.127s  coverage: 21.7% of statements

I introduced a new go library to do patch - bou.ke/monkey It can facilitate function/method patch like other interpreter language(i.e. python)