Add method names as constants

Example:
```go
	image := new(mocks.Image)
-	image.On("Add", mock.Anything)
+	image.On(mocks.ImageMethodAdd, mock.Anything)
```

```go
type Image struct {
	mock.Mock
}

+ const (
+ 	ImageMethodAdd = "Add"
+ 	ImageMethodGet = "Get"
+ )

func (_m *Image) Add(image model.Image) (uint64, error) {}
func (_m *Image) Get(settings storage.SettingsImage) ([]model.Image, error) {}
...
```