两个函数的设计。设计实现对文件 file.txt插入和增加的功能（1）实现文件中间插入一段话--------这是一个测试（2）在文件中添加数据。-The design of the two functions. The design and implementation of the file file.txt insert and increase functionality (1) file intervening passage-------- This is a test (2) add the data in the file.