Make sure multiple test base classes work for Explore

PR for https://issues.cask.co/browse/CDAP-729
In a unit-test context, if two test classes both extending TestBase have record scannable datasets, the initialization of the Hive Metastore for the second test class will not have the default databases created - for some reason. The quick fix is to manually recreate them at Explore start-up.
Build: https://builds.cask.co/browse/CDAP-RBT15
I moved all the Datasets definition classes in a new package in the explore tests, so that's why.