When creating temporary directories in tests, a timestamp and a global variable is not unique enough. You should always assume that the tests will be run in parallel. Even when the test runner you are using does not have built-in support for running them in parallel (though it could in the future), it's possible to run the same test suite multiple times in different processes (which may happen especially on CI servers - I once had tests conflicting because of listening on a fixed network port number).
