Handle events in the TestVM

[anorth]

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

[anorth]

@aarshkshah1992 I think you make have completed this. Please close the issue if you believe so, or leave a comment here explaining what still needs to be done.