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

まず、test_runner の mock 名前空間と、issue で説明されているファイルシステム API を確認します。mock.fileSystem.enable が設定済みファイルをどのように処理し、実際のファイルシステムにフォールバックし、オーバーライドモードをどのように扱うかを定義します。これらの動作が公開され、テストでカバーされていれば完了です。