mq: fix failing qrefresh test
Additions to test-mq-qrefresh.t in f08df4d38442 (mq: ignore subrepos
(issue2499)) were originally based on a version prior to d0e21c5fde41
(subrepo: handle missing subrepo spec file as removed). This fixes a
test failure that resulted from the former being applied after the
latter, noticed by abuehl.