Remove dependency on `go.uber.org/mock`

[AkihiroSuda]

https://github.com/search?q=repo%3Acontainerd%2Fnerdctl+go.uber.org%2Fmock&type=code

The tests that are using go.uber.org/mock should be rewritten to use plain structs and interfaces for simplicity.

[fahedouch]

we can use mockgen to generate mock interface implem and put then in seperate package. No need to redo the mockgen work.

[apostasie]

we can use mockgen to generate mock interface implem and put then in seperate package. No need to redo the mockgen work.

We would still depend on go.uber.org/mock (during test runs) though, right?

Or is there a different way to use mockgen?