Collect per-test coverage for both riak_test and eunit tests for giddyup

To enable us to be able to see the *combined* coverage of our unit and
integration tests, modify riak_test and the smoke_test runner to capture
coverage data per-test and post it as a giddyup artifact.

To maintain the current riak_test behaviour where the *combined*
coverage is reported on at the end of a run, each test writes its own
.coverdata file, cover is reset and then once all tests are run, the
coverdata files are all loaded and the total coverage is reported.