Add unit tests for artifacts commands path arguments expansion

In https://gitlab.com/gitlab-org/security/gitlab-runner/-/merge_requests/38 we started expanding artifacts path arguments in Go, rather than in the shell. To release https://gitlab.com/gitlab-org/security/gitlab-runner/-/merge_requests/38 quickly and on time this was tested only manually. Example line https://gitlab.com/gitlab-org/security/gitlab-runner/-/merge_requests/38/diffs#968cddea646e20e57b2edd3e8dc966cf4c268ab6_756_773