Text execution on Linux (*nix)

Currently test files are created using `tmpdir + "dir" + randomValue`.

On Linux (and most other *nix systems like Mac OS) `tmpdir` mostly yields `/tmp` (note no trailing slash) and above concatenation results in e.g. `/tmpdir0123456`. However, regularly you do not have access rights to write directly in the filesystem root and thus several of the tests fail. Writing in `/tmp/dir0123456` would work, tho.

The following pull request contains this fix with two additions:
* Changing from random folder numbers to a counter for the "number part" of the test directory. Doing so will prevent the tests from flickering.
* I've added another directory layer, so all test files are in one directory (makes deleting easier)

This results in e.g. `/tmp/st-tmp-dir/56`on my machine.

Something else:
In my test environments I usually clear such temp directories in a create method, so I am sure no files from previous test runs are present. Should we add this here as well?