Use real file system instead of mock-fs for testing

[mizdra]

Using mock-fs requires dirty hacks. In addition, it hides the defects that occur in the actual file system. Therefore, we will stop using mock-fs.

[mizdra]

I think it is good to test on the fixtures directory, like eslint.

https://github.com/eslint/eslint/blob/ce035e5fac632ba8d4f1860f92465f22d6b44d42/tests/lib/eslint/eslint.js#L303-L310