patched tests: do not report ModuleNotFoundWarning for the tests that are not present in a particular version of Python

rename test_patched_httpserver.py to test_patched_httpservers.py because there's test_httpservers.py not test_httpserver.py

--HG--
rename : greentest/test_patched_httpserver.py => greentest/test_patched_httpservers.py

test_hub_shutdown.py: check that shutdown() removes hub from threadlocal storage even if hub was never switched to

testrunner.py: do not report tracebacks listed in "Ignore tracebacks: " line in the test_???.py file

- if "Ignore tracebacks: " line is not present, ignore ExpectedException

Hub.shutdown: make sure current hub instance is not referenced by threadlocal variable even if it was never switched to