scsi: ufs: add long random read/write unit tests

Generalize the long_sequential_test framework to execute long random read
and long random write tests as well.
The random tests submit a large number of 4KB requests to randomly chosen
LBAs. Currently the tests are defined to read/write 64MB.

Change-Id: I9d797c0d61d863b2a9fbcd593a1be9065202ef76
Signed-off-by: Lee Susman <lsusman@codeaurora.org>