The path of the config file generated by testutils for completion tests
is passed as regular argument, not via COMP_WORDS. Use that config file
in complete_artifact() to ensure the test uses the right artifact
directory.

Since the artifact subcommand group has been created, we have been able
to tab complete both element names and artifact refs as arguments to
the artifact subcommands (e.g. bst artifact log), this commit adds a
test for this.