Add test cases for reload bug in riemann.config/index

`WrappedIndex`es are not working across reloads as they do not take into
account the fact that when an index already exists, the index inserted
into `next-core` is thrown away during a core merge.

This has the effect of breaking event expiration.
Merge pull request #446 from dlobue/add_wrapped-index_failure_test_case

Add test cases for reload bug in riemann.config/index