Drop duplicated tests from jdo/{datastore} packages

[andyjefferson]

We originally added jdo/hbase, jdo/spreadsheet, jdo/cassandra etc to give a simple way of testing other datastores since those datastores did not support all features and the test suite did not provide a way of easily disabling some of the main tests for particular datastores. We now allow that so jdo/general should be runnable (to some degree) with most datastores.

With this in mind we should aim to drop any duplicated tests from the jdo/{datastore} area where it is not datastore-specific.