When a `locale/*/gitlab.po` file is removed, the folder (*) will remain
on local dev machines as the folder still contains git ignored files.
This slows down the `gettext:regenerate` command, as the task looks for
the folders to determine which `gitlab.po` files need to be generated.

Add a step to remove these 'empty' folders before starting to regenerate
translations.

Add some specs for the gettext rake tasks.

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/238208