tests: allow application/x-troff-man as content-type for qcow2 files
This is reported for Fedora qcow2 files. While the issue was fixed on Fedora servers [1], Fedora mirrors may still serve qcow2 files with a wrong content-type. Hence, accept application/x-troff-man as content-type for qcow2 images for now.
[1] https://pagure.io/fedora-infrastructure/issue/10766
Followup of !470 (merged).