Fix TestStartup.testNNFailToStartOnReadOnlyNNDir test failure on Windows

org.apache.hadoop.hdfs.server.namenode.TestStartup.testNNFailToStartOnReadOnlyNNDir fails on Windows with below error message:

NN dir should be created after NN startup. expected:<[F:\short\hadoop-trunk-win\s\hadoop-hdfs-project\hadoop-hdfs\target\test\data\3\dfs\testNNFailToStartOnReadOnlyNNDir]name> but was:<[/F:/short/hadoop-trunk-win/s/hadoop-hdfs-project/hadoop-hdfs/target/test/data/3/dfs/testNNFailToStartOnReadOnlyNNDir/]name>

due to path not processed properly on Windows.