Shutdown graph properly in Infinispan graphdb test

Avoids the following spurious exception caused when merely clearing
the graph storage after a test completes without shutting the graph
down entirely:

Exception in thread "Thread-112" com.thinkaurelius.titan.core.TitanException: Could not acquire new ID block from storage
        at com.thinkaurelius.titan.graphdb.database.idassigner.StandardIDPool.renewBuffer(StandardIDPool.java:116)
        at com.thinkaurelius.titan.graphdb.database.idassigner.StandardIDPool.access$100(StandardIDPool.java:14)
        at com.thinkaurelius.titan.graphdb.database.idassigner.StandardIDPool$IDBlockThread.run(StandardIDPool.java:171)
Caused by: com.thinkaurelius.titan.diskstorage.PermanentStorageException: It seems there is a race-condition in the block application. If you have multiple Titan instances running on one physical machine, ensure that they have unique machine idAuthorities
        at com.thinkaurelius.titan.diskstorage.idmanagement.ConsistentKeyIDManager.getIDBlock(ConsistentKeyIDManager.java:122)
        at com.thinkaurelius.titan.graphdb.database.idassigner.StandardIDPool.renewBuffer(StandardIDPool.java:110)
        ... 2 more