Test envelope type enum with mixed index inheritance

[marcdaniels-toast]

Adds one unit test that belongs with recently-merged #1067. This checks the event envelope type enum for the index inheritance scenario where subtypes inherit from an indexed supertype. This is an important edge case to test since it validates that:

1. Concrete subtypes appear in the enum (even when they inherit)
2. Abstract supertypes don't appear (even when they're indexed)
3. The mix of inherited + own indexes works correctly