'which will be used by chunkservers during the tests to store any generated data' would describe the reason of providing them better. We can also write here, that:
* These directories may be located on one physical disk, by the long tests may not work in such a configuration (they generate a heavy traffic). It's OK for sanity checks.
* There should be about 60 GB of disk space available for the tests
* At least three directories have to be provided in order to run some of the test cases