Add option to allow tests in a directory to be explicitly skipped

Add skip option for use with the dir option to allow specified tests in a
directory to be skipped during a test run. This is useful in cases where a
directory contains a large number of tests of which a small number are only
appropriate to run in certain circumstances. A good example of such a case is
providing a way for open source users to build and run the RiakCS tests and
avoid the failure that would occur trying to run the replication tests.