Introduce a new test to identify regressions causing devices to go
missing on the system.

For each bus and class on the system the test checks the number of
devices present against a reference file, which needs to have been
generated by the program at a previous point on a known-good kernel, and
if there are missing devices they are reported.

Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>