Improvements to getPageTitle test-support helper

We probably should add comment block so IDEs could give hints of expected parameters OR provide TypeScript interface file. I think proper JSDoc for starters would be nice.

This helper does have some internal logic comments in it, we should probably remove it or separate it still for testing specifically this addon vs public helper.

The `Array.pop()` workaround for consumers is not needed, it's only applicable for tests in this repository.

_Originally posted by @raido in https://github.com/ember-cli/ember-page-title/issues/209#issuecomment-763450848_