libhdfs++: Clean up minidfscluster tests

A couple things need to be fixed with the minidfscluster tests

-Tests that are using hdfs_ext shouldn't be living in libhdfs-tests, any test in there should be able to be shared between both libraries.

-Tests added in HDFS-9890 relies on NDEBUG to turn on error simulation, ideally these should have some other switch so we can run error simulation on release builds.