It is used in `helper` package only, besides unit-tests.
Move unit-tests to the same package, where they belong.
Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>
`fmt.Errorf can be replaced with errors.New` and `fmt.Sprintf can be replaced with string addition`
Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>