Move eunit system tests to riak_test

The cluster startup and teardown in these eunit tests is rather fragile. The tests have been translated to riak_test, which is better at handling cluster configuration, in riak_test#143. This PR exports a function needed by those tests, removes the duplicates here, reworks some non-system tests to simpler unit tests (in riak_pipe_fitting), and also removes the cluster setup and teardown code. Once riak_test#143 is accepted, I suggest accepting this PR as well to reduce some of the fragility in our unit tests.