testing/test_basics: fix typo in Gateway mock

Fix:

```
testing/test_basics.py::TestPureChannel::test_factory_create[thread]
testing/test_basics.py::TestPureChannel::test_factory_getitem[thread]
testing/test_basics.py::TestPureChannel::test_channel_timeouterror[thread]
testing/test_basics.py::TestPureChannel::test_channel_makefile_incompatmode[thread]
  /home/ran/src/execnet/.tox/py310/lib/python3.10/site-packages/_pytest/unraisableexception.py:78: PytestUnraisableExceptionWarning: Exception ignored in: <function Channel.__del__ at 0x7fcb5b6a13f0>

  Traceback (most recent call last):
    File "/home/ran/src/execnet/execnet/gateway_base.py", line 605, in __del__
      self.gateway._send(msgcode, self.id)
  AttributeError: 'Gateway' object has no attribute '_send'. Did you mean: '_send_'?
```

Author: bluetech

testing/test_basics.py

@@ -297,7 +297,7 @@ class Gateway:
             def _trace(self, *args):
                 pass
 
-            def _send_(self, *k):
+            def _send(self, *k):
                 pass
 
         Gateway.execmodel = execmodel