Allow for finalizers as well as initializers

It would be extremely useful for individual tests to support finalizers, alongside the already-supported initializers. At the moment, one has to either (a) include the finalizer of the previous test in the initializer of the next, and thread the state between them globally, or (b) use GC finalizers.

Start by tracing the existing initializer support and how individual tests are represented. Define where a per-test finalizer runs relative to the next test, without requiring global state or GC finalizers. Add coverage showing that finalizers are supported alongside initializers and run reliably for each test.

Written by the indexing model from the issue text.