Testing/mocking 3rd party packages in Golang

I'm new to Golang and have been taking a TDD approach while learning the language. I've been getting along okay yet I find testing third party packages quite clunky, which leads me to believe that I've been taking the wrong approach.

The specific case I'm having trouble with is mocking a Redis client for error handling. The approach I've taken is to create my own interface, and the implementation wraps the clients methods that I want to use.

type Redis interface {
    Get(key string) (string, error)
}

type RedisClient struct {
    client *redis.Client
}

func (redisClient *RedisClient) New(client *redis.Client) *RedisClient {
    redisClient.client = client

    return redisClient
}

func (redisClient *RedisClient) Get(key string) (string, error) {
    return redisClient.client.Get(key).Result()
}

I can then create a mock which implements that same interface to return whichever values I specify, particularly for testing error handling.

I've hit a roadblock where a specific method on the client to perform transactions (MULTI) returns another interface belonging to that package. What would I do in this scenario? Implementing that interface myself seems out of the question.

Similarly, as usage of this client grows, my own implementation can grow to the point that it implements the whole interface to Redis - this seems to go against the whole idea of delegating this out to an external dependency.

Is there a better way to test third-party packages like this, for things such as error handling?