Breaks on old version to force lockstep upgrade, which should really be a depends-new-version (and can be switched around together with util-linux once the transition is finished). Using Breaks/Depends the 'wrong' way around is to make apt unpack things in the 'right' order (avoiding any gaps where /bin/su is not available during the upgrade phase).
A testsuite is also available. Instruction on how to run this testsuite are available in tests/README -- Balint Reczey <rbalint@ubuntu.com>, Sat, 12 Aug 2017 18:46:44 -0400