Add packet_test

[Bolodya1997]

Issue

Currently packet provider is not covered with test because there are no stubs for the packet API.

Solution

Create packet_test package with stubs. Ex:

type Option func(*packngo.Client)

func NewClient(options ...Option) *packngo.Client {
    client := new(packngo.Client)

    for _, option := range options {
        option(client)
    }

    return client
}

func WithDevices(conf *Config) Option {
    deviceService := newDeviceService(conf)
    return func(client *packngo.Client) {
        client.Devices = deviceService
    }
}

...