JMX APIs appear untested

Anything supported via JMX is part of the public API of the database.

Node tool uses JMX but doesn't seem to have its own unit test and the dtest nodetool_test.py is pretty sparse.

For values returned by JMX for the purposes of reporting we should test as best we can that we are getting "real" values end to end. Occasionally metrics end up with no values, or values in the wrong units.

For commands going the other direction they should be exercised. There is probably a lot of coverage of commands since they may be used when testing the features those commands are a part of so no need for duplication there.