Add failing test around index deletion

This test demonstrates that currently Yokozuna fails to cleanup the
instance dir of an index after it has been deleted.
Cleanup index on delete

When an index is deleted in Yokozuna its instance dir containing
configuration and actual index files should be deleted.  The first
step of deletion is to remove the index from the list of indexes in
the ring.  This is done on the node the receives the delete request.
This removal from the ring is then propagated to the other nodes via
gossip.  As nodes notice index removal events in `yz_events` a
`local_remove` function is called that is responsible for sending the
delete request to the local Solr instance.  But first it checks if the
index exists.  If it goes it simply does nothing.

The issue is that `yz_index:exists` returns true if and only if the
index is both a) in the ring and b) pingable from Solr.  The problem
is the index was already removed from the ring by the time
`local_remove` is called and thus `exists` will always return false
and the actual Solr Core will never be deleted.

This patch changes `local_remove` to only consider if the Solr Core is
still pingable.
Merge pull request #99 from basho/rz-index-remove

cleanup index on delete