Fix GNOME GitLab CI test job failures because of missing /dev entries
From 23-Feb-2020 onwards, GNOME GitLab Continuous Integration test jobs have been failing running unit tests which previously succeeded.
This is to address repeatable failures seen with test_BlockSpecial and
test_SupportedFileSystems noted in
!55 (comment 725245).
It is not addressing sporadic failures occassionally seen with
test_PipeCapture and test_SupportedFileSystems first noted in
!51 (comment 659145).