Simpletest leaves files behind

If you run Simpletests locally, you have probably noticed that it leaves a lot of empty directories behind, e.g. sites/default/files/simpletest/12345 (with different numbers every time). These are not removed by pressing the “Clean environment” button.

This regression was introduced in #443200: User pictures does not work with private files (by me - oops) and in #517814: File API Stream Wrapper Conversion, because the patches for these issues were made a long time ago and then rerolled without noticing the change in #512104: Simpletest breaks high volume sites.

This patch also addresses the issue raised in #555364: Set a testing temp directory as well as public/private.