Move fsPromises.readdir test cases under fs.readdir group into fsPromises.readdir group

[ywpark1]

I just checked the fsPromises.readdir test cases under fs.readdir.spec.js file.

Currently, there are fsPromises.readdir test cases with '(promise)' prefix under fs.readdir group. I can see that there is alslo fsPromises.readdir test group below.

I think it is better to move fsPromises.readdir test cases into fsPromises.readdir group for consistency. I would like to work on this issue if it is ok.