bpo-30107: don't dump core on expected test_io crash (#1235) (#1241)

bpo-30107: don't dump core on expected test_io crash (#1235) (#1241)

test_io has two unit tests which trigger a deadlock:

* test_daemon_threads_shutdown_stdout_deadlock()
* test_daemon_threads_shutdown_stderr_deadlock()

These tests call Py_FatalError() if the expected bug is triggered
which calls abort(). Use test.support.SuppressCrashReport to prevent
the creation on a core dump, to fix the warning:

Warning -- files was modified by test_io
  Before: []
  After:  ['python.core']
(cherry picked from commit 2a1aed04b0943636f605543522e16cca1dc23e70)