bpo-30908: Fix dangling thread in test_os.TestSendfile (#2680) (#2845)

bpo-30908: Fix dangling thread in test_os.TestSendfile (#2680) (#2845)

tearDown() now clears explicitly the self.server variable to make
sure that the thread is completely cleared when tearDownClass()
checks if all threads have been cleaned up.

Fix the following warning:

$ ./python -m test --fail-env-changed -m test.test_os.TestSendfile.test_keywords -R 3:1 test_os
(...)
Warning -- threading_cleanup() failed to cleanup 0 threads after 3 sec (count: 0, dangling: 2)
(...)
Tests result: ENV CHANGED
(cherry picked from commit d1cc037d1442cc35d1b194ec8e50901514360949)