tests: update more of test-bdiff.py to use unittest (part 2 of 4)

tests: migrate test-bdiff.py to use unittest (part 1 of 4) This moves all the test() calls, which were easy and mechanical.

import-checker: do not enforce lexical sort accross stdlib/local boundary Before this change, you could get in a start where the checker would either complain about importing local module before stdlib one or complain about the local one being wrongly lexically sorted with the stdlib one. We detect the boundary and avoid complaining about lexical sort across it.

cg1packer: fix `compressed` method `cg1packer.compressed()` returns True even if `self._type` is 'UN'. This patch fixes it.