bpo-37069: tests use catch_unraisable_exception() (GH-13762)

Modify test_coroutines, test_cprofile, test_generators, test_raise,
test_ssl and test_yield_from to use
support.catch_unraisable_exception() rather than
support.captured_stderr().

test_thread: remove test_save_exception_state_on_error() which is now
updated. test_unraisable_exception() checks that sys.unraisablehook()
is called to handle _thread.start_new_thread() exception.

test_cprofile now rely on unittest for test discovery: replace
support.run_unittest() with unittest.main().