[IMP] running speed of some tests & new testcase type

Some tests (e.g. mail) have expensive and significant DB setup for a
number of small and cheap tests. Using a TransactionCase, the DB setup
far dominates the tests themselves, by up to 10x (mail unit tests take
~130s on my machine, the tests themselves take ~15s).

The SavepointCase introduced here is an hybrid of SingleTransactionCase
and TransactionCase: it uses a single transaction for all tests in a
class, but each test case is isolated by a rollbacked savepoint. This
allows a common DB setup (via setUpClass) while keeping independent
tests.

TransactionCase should remain the primary test case superclass, but
SavepointCase can be a fair optimisation when setup costs far dominate.

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