Annotate riak_test modules for cover specification

[kellymclaughlin]

@Vagabond has made a change in #579 that allows each test module to specify which applications or modules the test is intended to cover. Now the annotations need to be added to each test module so that we can get an accurate picture of where we stand in terms of test coverage.

The annotations are documented in the riak_test README and take the following form for applications:

-cover_apps([riak_core, riak_kv]).

Similarly for modules:

-cover_modules([riak_kv_bitcask_backend, riak_core_ring]).