Automatically enable handlers if they are required for a specific test.

[jmalloc]

Blocked by #197

If a test is found to be "possible" as per #197, we will know which handlers are required in order for it to be possible, so we can enable them automatically.

This would mean you'd only need to use Test.EnableHandlers() when testing projections and integrations that don't produce any events.

[jmalloc]

This idea might not be very useful because we wont be able to deduce this information in Prepare() (because we don't know what messages a future Expect() call will care about) but I'd like to explore the idea before dismissing it.