2 cases in test_device.py failed on FreeBSD

[yzgyyang]

tests/comparators/test_device.py::test_diff FAILED
tests/comparators/test_device.py::test_diff_reverse FAILED

The reason being that the expected output assumed that /dev/null should always have .major:1 .minor:3, however, in FreeBSD,

➜  ~ python3
Python 3.6.2 (default, Jul 18 2017, 01:40:45) 
[GCC 4.2.1 Compatible FreeBSD Clang 4.0.0 (tags/RELEASE_400/final 297347)] on freebsd12
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> st = os.lstat('/dev/null')
>>> os.major(st.st_rdev)
0
>>> os.minor(st.st_rdev)
14

[yzgyyang]

Fixed in a5d7a19. A patch is also been created:
diffoscope_test_device_patch.diff.txt

@emaste @mapreri

[yzgyyang]

Committed here.