…n subdirectory

Fix cleanup after failing tests by moving cleanup into new tearDown() method: old cleanup code was never reached when assertions fail. consecutive runs of PHPUnit resulted in many failing tests e.g. when foo/ was still present.

We're using tearDown() instead of try..finally in each test because its more concise. next task is to move all test files into one common base directory, so simplify the cleanup and make it more future proof when using other files than currently considered in cleanup.

Make cleanup more stable by moving all files created during tests from __DIR__ to new subfolder __DIR__/tmp by replacing __DIR__ with static::TEMP_DIR, which is defined as subfolder __DIR__/tmp.

…and later to class variable

…rted in PHP 5.6 and later to ordinary member variable