Update Riak Search migration test

Get the RS migration test mostly working again.

* Revert the change to Yokozuna to only respect `yz_index` on
  non-default bucket types. Just because `search=true` and Yokozuna is
  enabled doesn't mean an index is created named after that bucket. So
  migration becomes really weird if it's implicit like that. Instead I
  reinstated association on any type so that migration of RS to
  Yokozuna is explicit. This also means the index may be named
  something other than the bucket.

* The Yokozuna AAE trees must be manually cleared after associating
  the index if relying on AAE for migration. The is no longer any
  automatic AAE clearing in Yokozuna. It was removed in commit
  `786c3f2`.

* Move AAE riak test helpers into `yz_rt` since they are applicable to
  multiple tests.
Strip RS hooks from bucket properties

If Riak Search is not enabled make sure to strip the pre-commit hook
from all bucket's properties. This shouldn't be necessary but there
was a bug in previous versions of Riak that leaked bucket fixups into
the raw ring.
Remove unused functions
Fix type spec
Merge remote-tracking branch 'origin/develop' into test/rz/rs-migration-3
Move RS migration code to separate module
Merge branch 'test/rz/rs-migration-3' into develop

Conflicts:
	src/yz_wm_search.erl
Rename migration test

The migration test needed to be renamed for riak_test to consider it a
test because now there is a regular module with the same name.