test_runner: support mock file system

What is the problem this feature will solve?

The current test requires writing files to a real disk when reading and writing files related to the file system, which is very unsafe.

After all, the file may remain due to test case failure and other reasons.

What is the feature you are proposing to solve the problem?

Under the mock namespace, APIs are exposed to provide a mock file system.

mock.fileSystem.enable({ });

And provide some options

e.g.

Override the file that defined by defaults

mock.fileSystem.enable({
  files: {
    '/path/to/file': new Uint8Array([0])
  }
});

// it will read the virtual file system first if it does exist
// if not found, then read the read the real file system
fs.readFileSync('/path/to/file')

Override the whole file system

mock.fileSystem.enable({
  files: {
    '/path/to/file': new Uint8Array([0])
  },
  override: true // override the whole file system
});

// This file is exists in the real file system
// But not define in the vitual file system, so it should throw an error
fs.readFileSync('/etc/proc')

What alternatives have you considered?

mock-fs

首先检查 test_runner 的 mock 命名空间以及 issue 中描述的文件系统 API。明确 mock.fileSystem.enable 如何处理已配置的文件、回退到真实文件系统以及处理覆盖模式；当这些行为已对外提供并由测试覆盖时，即表示完成。