migrate riak_kv's mapred_test suite

The mapred_test eunit suite had a fragile setup stage that frequently
caused the automated builders to fail. It should be more stable here.

The suite has also been broken into several independent tests, to make
it possible to run each major piece individually.
don't import rt_pipe.hrl until it's merged from the other branch
remove redundant assert

this is already handled by rt:build_cluster; it made it here through
copy/paste from old tests
remove unnecessary unlink from riakc_pb_socket

the process stops 'normal' so there's no problem with it being linked
build 3-node clusters for mapred tests (instead of 1 node)

may as well take advantage of riak_test's ease of building clusters
Merge pull request #209 from branch 'bwf-move-mapred-test'