Add unit test for NameNode failing to start when name dir is unwritable

This can be placed in org.apache.hadoop.hdfs.server.namenode.TestStartup test class.

UPDATE: this JIRA is for name dir only; for the edit log directories, we have unit test TestInitializeSharedEdits#testInitializeSharedEdits, which tests that in HA mode, we should not have been able to start any NN without shared dir.