Add tests for dm-writeboost

dm-writeboost is a caching driver like dm-cache, bcache and enhance-io.
This patch introduces the minimum stack and tests for writeboost driver.

Now,
`dmtest list --suite writeboost` shows

writeboost
  WriteboostTests
    fio_cache
    fio_database_funtime
    fio_sub_volume
Merge pull request #3 from akiradeveloper/feature/add-writeboost-test

Add tests for dm-writeboost