completions.py: Add a test for our artifact ref autocompletions
Description
Since the addition of the bst artifact log
command (!920 (merged)), we have been able to tab-complete artifact refs that exist in the local cache.
This MR adds a test for this.
Note that this test is different from the other completion tests in the sense that we must build the artifact before we try to auto-complete its ref
Edited by James Ennis