[Hot Fix] Fix WriteAheadLogBackedBlockRDDSuite

WriteAheadLogBackedBlockRDDSuite failed due to random string file name
```
/tmp/1418645532799-0/??????????
[info] - Read data available only in write ahead log, and test storing in block manager *** FAILED *** (2 milliseconds)
```
Using another way to produce random file name.