Add a test for promises.rename() for renaming an existing file

[Michael-Overall]

Current tests in fs.rename.spec.js do not include a test to check if fs.promises.rename() successfully renames an existing file.

The new test could be based on logic for the equivalent existing Async test:

 it('should rename an existing file', function(done) { //...

and possibly logic found for rename in an existing Node.js test

I'd like to make this change if it seems reasonable, and add it to the existing test suite in fs.rename.spec.js

[humphd]

It's all yours @Michael-Overall, go for it. I'd suggest that you start by adding just one test for this to the same file we already have. There are probably a bunch of rename tests we could do. Just pick one.