Test libhdfs++ with existing libhdfs tests

Leverage existing libhdfs tests, such as test_libhdfs_threaded.c, to exercise functionality implemented in libhdfs++. The remainder of functionality needed to run the tests can be supported by delegating to libhdfs. This allows functions implemented in libhdfs++ to be tested prior to implementing the entirety of the hdfs c-api (i.e. reads can be tested prior to implementing writes).