Incorrect HFile path in TestHFilePerformance print output (fix for easier debugging)

While trying to look at the issue in HBASE-12866, came across a minor issue in TestHFilePerformance while printing the HFile path. This has a directory path delimiter missing. It is only a sysout issue but can be fixed for easier debugging. So basically in timewrite function System.out.println(ROOT_DIR + path.getName());
should be System.out.println(ROOT_DIR + "/" + path.getName());

so that current output
ROOT_DIR/target/test-data/.../TestHFilePerformanceHFile.Performance
is fixed as
ROOT_DIR/target/test-data/.../TestHFilePerformance/HFile.Performance