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. ```js mock.fileSystem.enable({ }); ``` And provide some options e.g. Override the file that defined by defaults ```js 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 ```js 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](https://www.npmjs.com/package/mock-fs)