Improve unit tests for pkg/registry/client.go

[chengfang]

Describe the bug According to the test coverage report at https://app.codecov.io/gh/argoproj-labs/argocd-image-updater/blob/master/pkg%2Fregistry%2Fclient.go , some important code paths are not covered in function:

func (client *registryClient) TagMetadata(manifest distribution.Manifest, opts *options.ManifestOptions) (*tag.TagInfo, error) {

Need to add tests for those code paths in https://github.com/argoproj-labs/argocd-image-updater/blob/master/pkg/registry/client.go#L198