commitlog_tests: test cleanup
commitlog_tests: test cleanup

This cleanup patch got lost in git-space some time ago. It is however sorely
needed...

* Use cleaner wrapper for creating temp dir + commit log, avoiding
  having to clear and clean in every test, etc.
* Remove assertions based on file system checks, since these are not
  valid due to both the async nature of the CL, and more to the point,
  because of pre-allocation of files and file blocks. Use CL
  counters/methods instead
* Fix some race conditions to ensure tests are safe(r)
* Speed up some tests

duarten/scylla,avikivity/scylla,raphaelsc/scylla,scylladb/scylla,scylladb/scylla,raphaelsc/scylla,kjniemi/scylla,kjniemi/scylla,kjniemi/scylla,duarten/scylla,avikivity/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,raphaelsc/scylla,scylladb/scylla