xTimer hang test

This is a runtime test for the RIOT xtimer implementation it prints text Testing... (<percentage>) (approximately) every TEST_INTERVAL_MS ms (milliseconds) for an overall test duration of TEST_TIME_S seconds.

The test will print [SUCCESS] on completion, if it stops or hangs before, the test has failed.

Please note (again), this is a runtime test to check if xtimer runs into a deadlock, it is not about clock stability nor accuracy of timing intervals.