Add test for optional -s after 'GitLabber'

The following discussion from gitlab-development-kit!1697 (merged) should be addressed:

• @aqualls started a discussion:

  Oooo, I thought that word looked odd. Substitutions.yml tests for GitLabber but doesn't include an optional check for an 's' at the end, so this instance was missed. I don't want to impede merging this work, though, so I'll move this change to a follow-up issue.