tests/test-chg.t

tests: enable import checker for tests/**.py files

Several known-bad files are excluded as they couldn't be trivially fixed.
In principle, we should fix them first, however, it would have more risk
to keep Py3k porting going without the test coverage.

Still contrib/**.py aren't covered, which needs another round.

init repo

  $ hg init foo
  $ cd foo

ill-formed config

  $ hg status
  $ echo '=brokenconfig' >> $HGRCPATH
  $ hg status
  hg: parse error at * (glob)
  [255]