Properly clean up db bench dirs after bench is done.

[glyh]

As title.

[georgeee]

I suggest that we go one step further and clean the directory used for benchmark at the end of each test

In order to do that let's use the same directory for all of the tests without suffixing it with the name. In the initialization code let's check if that directory exists and if it does, let's remove it.

This way we'll ensure that the test can be run if there is enough disk space for just one directory because it's much lighter requirement than having disk space for 8 directories 20 GB each.

So, in the initialization code for each test we'll remove the directory from the previous run if it existed. After all the tests are run (exactly as it is implemented right now), let's clean up the directory that remained after the last test.

[glyh]

!ci-build-me