Establish policy for writing regression tests

Ideally, every non-trivial bug fix will have a regression test. Consider making and documenting a policy that requires that bug fixes will have accompanying regression tests, or some searchable justification (Trac keyword?) for why it's infeasible to add one. (Often this is a symptom that the underlying code is too complex and needs refactoring.)

This documentation should probably be somewhere under doc/HACKING.