Handle events in the TestVM

The integration TestVM doesn't handle actor events. PR #1320 collects them in invocation context, but does not make them available for inspection by test code. Consider some of the following paths:
- Add events to `MessageResult`, as the are present in the top-level message receipt on chain
- Add events to entries in the InvocationTrace, where their provenance can be inspected in more detail

Either of these should be coordinated with ongoing efforts to provide an abstract integration testing interface that can support testing on either a real VM or the TestVM (see #1236 and others).

FYI @Stebalien @alexytsu