Use path.join instead of string interpolation

[alyip98]

Assuming forward slashes may cause errors on windows systems

What is the purpose of this pull request? (put "X" next to an item, remove the rest)
• [x] Bug fix

What is the rationale for this request?
This line was causing a error in tests as createDir uses string splitting based on the path.sep native to the OS.

function createDir(pathArg) {
  const { dir, ext } = path.parse(pathArg);
  const dirNames = (ext === '')
    ? pathArg.split(path.sep)
    : dir.split(pathArg.sep);

What changes did you make? (Give an overview)
Changed a path concatenation to use path.join instead of string interpolation.

It was the only instance I could find in the entire project that was using string interpolation.

Provide some example code that this change will affect:
Should not have any changes

Is there anything you'd like reviewers to focus on?

Testing instructions:

Proposed commit message: (wrap lines at 72 characters)
Assuming forward slashes may cause errors on windows systems.
Let's use path.join to maintain cross platform compatibility.

[openorclose]

LGTM 🚀