Add tests for dm-writeboost

Hi, Joe,
this is the initial commit of the testing plugin for dm-writeboost.
I am working under dmtest/tests/writeboost that is segregated from other codebase.
If I am not going the wrong direction please git-pull.


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