libhdfs++: test4tests doesn't detect C++ test changes

All patches "fail" because test4tests doesn't look for new or updated C++ tests. This isn't a huge deal as long as everyone is careful with their patches and reviews, but I think this would be really nice to have prior to integrating into the mainline branch.

Not sure if it's worth doing before rebasing onto a newer version of trunk in case there's any build system changes that'd break the patch. I'd be thrilled if anyone who knows the maven and CI infrastructure well could give some pointers about how to go about extending the test framework.