tests: Fix /umockdev-testbed/dev_query_gudev for systemd 251
tests: Fix /umockdev-testbed/dev_query_gudev for systemd 251

251rc2 has a stricter creation of an sd-device object [1].  In
particular, it calls sd_device_get_devnum() (which reads MAJOR and MINOR
from the uevent file) and compares it to the major/minor from devfs.

Supply these two properties in the test case. At some point umockdev
could create these automatically from the `dev` attribute.

[1] https://github.com/systemd/systemd/commit/a46f9cd049a

martinpitt/umockdev,martinpitt/umockdev,martinpitt/umockdev