fix issue#392-- add test for promises.rename() to rename existing file

[Michael-Overall]

Adds test for promises.rename() to tests/spec/fs.rename,spec.js to check renaming an existing file.

Closely mimics logic for an existing test that checks if the the Async rename() function renames an existing file (so no fs.promises functions are used for open(), close(), or stat() ).

Closes #392

/cc @humphd