「"golang" test "os.Stdin" "keinos"」でググってもヒットしなかったので、自分のググラビリティとして。
In the dependency injection point of view, it is a good practice to var OsStdin = os.Stdin and use OsStdin instead of os.Stdin. Then monkey patch (temporary replace) the variable during the test.
But if the external package doesn't support that OsStdin alias feature, and uses os.Stdin, we need to mock the os.Stdin some how.
