asyncio: Fix BaseSelectorEventLoopTests (#4595)

Currently, two tests fail with PYTHONASYNCIODEBUG=1 (or using -X
dev).
diff --git a/Lib/test/test_asyncio/test_selector_events.py b/Lib/test/test_asyncio/test_selector_events.py
index e67afcc..2adb43a 100644
--- a/Lib/test/test_asyncio/test_selector_events.py
+++ b/Lib/test/test_asyncio/test_selector_events.py
@@ -187,6 +187,7 @@
         sock = mock.Mock()
         sock.fileno.return_value = 10
         sock.recv.side_effect = BlockingIOError
+        sock.gettimeout.return_value = 0.0
 
         self.loop.add_reader = mock.Mock()
         self.loop.remove_reader = mock.Mock()
@@ -267,6 +268,7 @@
         sock = mock.Mock()
         sock.fileno.return_value = 10
         sock.send.side_effect = BlockingIOError
+        sock.gettimeout.return_value = 0.0
 
         self.loop.add_writer = mock.Mock()
         self.loop.remove_writer = mock.Mock()