Increase coverage (#42)

* Introduce mocking framework and increase coverage (#22)

This change introduces two common golang patterns:

- types: this will allow fine-tuned control over imported
types by defining where they will be used and how

- mock: this allows the generation of mock constructors,
which allows for testing any individual path in a method by
"injecting" a mock method which matches the expected type

This change also increases test coverage to 100%

Co-authored-by: Yoshiyuki Mineo <Yoshiyuki.Mineo@jp.sony.com>

* gofmt

---------

Co-authored-by: Bradley Boutcher <btboutcher@icloud.com>

mock/sonyflake_mock.go

@@ -0,0 +1,35 @@
+// Package mock offers implementations of interfaces defined in types.go
+// This allows complete control over input / output for any given method that consumes
+// a given type
+package mock
+
+import (
+	"fmt"
+	"net"
+
+	"github.com/sony/sonyflake/types"
+)
+
+// NewSuccessfulInterfaceAddrs returns a single private IP address
+func NewSuccessfulInterfaceAddrs() types.InterfaceAddrs {
+	ifat := make([]net.Addr, 0, 1)
+	ifat = append(ifat, &net.IPNet{IP: []byte{192, 168, 0, 1}, Mask: []byte{255, 0, 0, 0}})
+
+	return func() ([]net.Addr, error) {
+		return ifat, nil
+	}
+}
+
+// NewFailingInterfaceAddrs returns an error
+func NewFailingInterfaceAddrs() types.InterfaceAddrs {
+	return func() ([]net.Addr, error) {
+		return nil, fmt.Errorf("test error")
+	}
+}
+
+// NewFailingInterfaceAddrs returns an empty slice of addresses
+func NewNilInterfaceAddrs() types.InterfaceAddrs {
+	return func() ([]net.Addr, error) {
+		return []net.Addr{}, nil
+	}
+}