CDK-128: Adding FS tests for new repo API methods.
CDK-128: Extracting reusable DFS test code.

The new MiniDFSTest superclass sets up and tears down a MiniDFSCluster and
makes both local and distributed FileSystems available via getDFS and getFS.
CDK-128: Updating FS Dataset tests to run in a DFS.
CDK-128: Adding much more testing for core Readers.

TestDatasetReaderCommon now has tests that should pass for all DatasetReaders,
and can be extended by implementers to verify new DatasetReaders. The other
*Common tests apply these tests to the single and multiple file readers.

Also added a lot of new tests for error cases and some convenience methods in
DatasetTestUtilities. Found a few bugs, which are fixed and included.