Add an unit test framework for kubekey

[lshmouse]

Currently, the test coverity is very low. We need to add an unit test framework for KK command to simulate all kinds of cases

$ go test ./... -coverprofile cover.out 
?   	github.com/kubesphere/kubekey	[no test files]
?   	github.com/kubesphere/kubekey/apis/kubekey/v1alpha1	[no test files]
?   	github.com/kubesphere/kubekey/clients/clientset/versioned	[no test files]
?   	github.com/kubesphere/kubekey/clients/clientset/versioned/fake	[no test files]
?   	github.com/kubesphere/kubekey/clients/clientset/versioned/scheme	[no test files]
?   	github.com/kubesphere/kubekey/clients/clientset/versioned/typed/kubekey/v1alpha1	[no test files]
?   	github.com/kubesphere/kubekey/clients/clientset/versioned/typed/kubekey/v1alpha1/fake	[no test files]
?   	github.com/kubesphere/kubekey/clients/informers/externalversions	[no test files]
?   	github.com/kubesphere/kubekey/clients/informers/externalversions/internalinterfaces	[no test files]
?   	github.com/kubesphere/kubekey/clients/informers/externalversions/kubekey	[no test files]
?   	github.com/kubesphere/kubekey/clients/informers/externalversions/kubekey/v1alpha1	[no test files]
?   	github.com/kubesphere/kubekey/clients/listers/kubekey/v1alpha1	[no test files]
# github.com/kubesphere/kubekey/pkg/cluster/upgrade
pkg/cluster/upgrade/precheck.go:114:4: Println arg list ends with redundant newline
?   	github.com/kubesphere/kubekey/cmd	[no test files]
?   	github.com/kubesphere/kubekey/cmd/kk	[no test files]
ok  	github.com/kubesphere/kubekey/controllers/kubekey	3.978s	coverage: 0.0% of statements
?   	github.com/kubesphere/kubekey/pkg/addons	[no test files]
?   	github.com/kubesphere/kubekey/pkg/addons/charts	[no test files]
?   	github.com/kubesphere/kubekey/pkg/addons/manifests	[no test files]
?   	github.com/kubesphere/kubekey/pkg/bootstrap/configuration	[no test files]
?   	github.com/kubesphere/kubekey/pkg/bootstrap/dependencies	[no test files]
?   	github.com/kubesphere/kubekey/pkg/bootstrap/registry	[no test files]
?   	github.com/kubesphere/kubekey/pkg/cluster/add	[no test files]
?   	github.com/kubesphere/kubekey/pkg/cluster/certs	[no test files]
?   	github.com/kubesphere/kubekey/pkg/cluster/delete	[no test files]
?   	github.com/kubesphere/kubekey/pkg/cluster/install	[no test files]
ok  	github.com/kubesphere/kubekey/pkg/kubernetes/preinstall	0.006s	coverage: 3.6% of statements

[FeynmanZhou]

Thanks for pointing this issue. Would you like to help us to add a unit test framework and contribute to KubeKey? It would be much appreciated that you can work with us to complete it.