Destroy nodes after node creation tests.

The test teardown method is calling rclpy.shutdown() before the nodes
created here are destroyed. With the inclusion of parameter services
this is causing the tests to hang after printing the following
exceptions:

    ========================================================================================================================== 7 passed in 0.11 seconds
    Exception ignored in: <bound method Node.__del__ of <rclpy.node.Node object at 0x7fc321e0fd30>>
    Traceback (most recent call last):
      File "/tmp/scree/src/rclpy-mount/rclpy/rclpy/node.py", line 410, in __del__
        self.destroy_node()
      File "/tmp/scree/src/rclpy-mount/rclpy/rclpy/node.py", line 353, in destroy_node
        _rclpy.rclpy_destroy_node_entity(srv.service_handle, self.handle)
    RuntimeError: Failed to fini 'rcl_service_t': rcl node is invalid, rcl instance id does not match, at /tmp/scree/src/ros2/rcl/rcl/src/rcl/node.c:461
    Exception ignored in: <bound method Node.__del__ of <rclpy.node.Node object at 0x7fc320597828>>
    Traceback (most recent call last):
      File "/tmp/scree/src/rclpy-mount/rclpy/rclpy/node.py", line 410, in __del__
        self.destroy_node()
      File "/tmp/scree/src/rclpy-mount/rclpy/rclpy/node.py", line 353, in destroy_node
        _rclpy.rclpy_destroy_node_entity(srv.service_handle, self.handle)
    RuntimeError: Failed to fini 'rcl_service_t': rcl node is invalid, rcl instance id does not match, at /tmp/scree/src/ros2/rcl/rcl/src/rcl/node.c:461
    Exception ignored in: <bound method Node.__del__ of <rclpy.node.Node object at 0x7fc320597b70>>
    Traceback (most recent call last):
      File "/tmp/scree/src/rclpy-mount/rclpy/rclpy/node.py", line 410, in __del__
        self.destroy_node()
      File "/tmp/scree/src/rclpy-mount/rclpy/rclpy/node.py", line 353, in destroy_node
        _rclpy.rclpy_destroy_node_entity(srv.service_handle, self.handle)
    RuntimeError: Failed to fini 'rcl_service_t': rcl node is invalid, rcl instance id does not match, at /tmp/scree/src/ros2/rcl/rcl/src/rcl/node.c:461

Destroying the nodes before the teardown runs resolves both the
indefinite hang and the exceptions.

rclpy/test/test_create_node.py

@@ -32,24 +32,26 @@ def tearDownClass(cls):
 
     def test_create_node(self):
         node_name = 'create_node_test'
-        rclpy.create_node(node_name)
+        rclpy.create_node(node_name).destroy_node()
 
     def test_create_node_with_namespace(self):
         node_name = 'create_node_test'
         namespace = '/ns'
-        rclpy.create_node(node_name, namespace=namespace)
+        rclpy.create_node(node_name, namespace=namespace).destroy_node()
 
     def test_create_node_with_empty_namespace(self):
         node_name = 'create_node_test'
         namespace = ''
         node = rclpy.create_node(node_name, namespace=namespace)
         self.assertEqual('/', node.get_namespace())
+        node.destroy_node()
 
     def test_create_node_with_relative_namespace(self):
         node_name = 'create_node_test'
         namespace = 'ns'
         node = rclpy.create_node(node_name, namespace=namespace)
         self.assertEqual('/ns', node.get_namespace())
+        node.destroy_node()
 
     def test_create_node_invalid_name(self):
         node_name = 'create_node_test_invalid_name?'