convert riak_pipe eunit system tests to riak_test

This will allow us to remove the fragile setup/teardown logic in the
riak_pipe module.

pipe_verify_examples <- half of riak_pipe:basic_test_
pipe_verify_basics <- other half of riak_pipe:basic_test_
pipe_verify_exceptions <- riak_pipe:exception_test_
pipe_verify_handof <- riak_pipe:limits_test_
pipe_verify_sink_types <- riak_pipe:sink_type_test_
use an intercept instead of riak_core_tracer in pipe handoff test

riak_core_tracer was not providing accurate operation counts, but
riak_pipe_vnode_intercept seems to
retry non-deterministic tests 10 times on failure

we may be able to do something smarter for
pipe_verify_exceptions:verify_queue_limit, like a coordinating process
as used in pipe_verify_handoff, but do the simple thing for now
break pipe restart input forwarding into its own test

The input-forward-after-restart-failure test must run on a single node,
but it's nice to be able to run the others in pipe_verify_exceptions on
multi-node clusters. This commit splits them into different tests to
make that possible.
Merge pull request #143 from branch 'bwf-pipe'