test(v2): refactor and modernize tests, improve error assertions, and update mocks for v2

Author: YoshiyukiMineo

README.md

@@ -18,10 +18,10 @@ As a result, Sonyflake has the following advantages and disadvantages:
 
 - The lifetime (174 years) is longer than that of Snowflake (69 years)
 - It can work in more distributed machines (2^16) than Snowflake (2^10)
-- It can generate 2^8 IDs per 10 msec at most in a single machine/thread (fewer than Snowflake)
+- It can generate 2^8 IDs per 10 msec at most in a single instance (fewer than Snowflake)
 
 However, if you want more generation rate in a single host,
-you can easily run multiple Sonyflake ID generators concurrently using goroutines.
+you can easily run multiple Sonyflake instances parallelly using goroutines.
 
 Installation
 ------------

v2/mock/sonyflake_mock.go

@@ -3,7 +3,7 @@
 package mock
 
 import (
-	"fmt"
+	"errors"
 	"net"
 
 	"github.com/sony/sonyflake/v2/types"
@@ -19,10 +19,12 @@ func NewSuccessfulInterfaceAddrs() types.InterfaceAddrs {
 	}
 }
 
+var ErrFailedToGetAddresses = errors.New("failed to get addresses")
+
 // NewFailingInterfaceAddrs returns an error.
 func NewFailingInterfaceAddrs() types.InterfaceAddrs {
 	return func() ([]net.Addr, error) {
-		return nil, fmt.Errorf("test error")
+		return nil, ErrFailedToGetAddresses
 	}
 }