bpo-33723: Fix test_time.test_thread_time() (GH-8267)

The test failed on AMD64 Debian root 3.x buildbot because the busy
loop of 100 ms only increased time.thread_time() by 19.9 ms which is
smaller than 20 ms. Modify the test to tolerate a delta of at least
15 ms instead of 20 ms.