test: Run tests in unique directory

The tests currently create various files and directories with generic names and manipulate with them directly in the /tmp directory. The tests also remove all files in /tmp with the generic prefixes like move, copy, etc. This can simply cause test suite failures if files with such names already exist in the /tmp directory. Also, the test suite can easily remove files created by other applications. Let's always create an unique directory in /tmp for tests to fix the mentioned problems...