Don't change britney.conf from the source tree and restore it back after the
tests; this is prone to leave a broken config behind if a test gets
interrupted. Instead, copy the file to our temp dir, and let tests hack on it
there.
(Introduced in lp:~canonical-ci-engineering/britney/boottesting-support)
- Added docstrings
- Replaced format by %
- Import ADT_EXCUSES_LABELS from autopkgtest.py instead of redefining it
- Renamed test to avoid name conflict with autopkgtest.py from britney
- Fixed some formatting