Fix broken test due to BUSYBOX -> TEST_IMG rename

The BUSYBOX variable was renamed to TEST_IMG in
54b48a9, however
0ddf428 got merged
after that change, but was out of date, and therefore
caused the tests to fail:

```
=================================== FAILURES ===================================
________ ServiceTest.test_create_service_with_network_attachment_config ________
tests/integration/api_service_test.py:379: in test_create_service_with_network_attachment_config
    container_spec = docker.types.ContainerSpec(BUSYBOX, ['true'])
E   NameError: global name 'BUSYBOX' is not defined
```

Fix the test by using the correct variable name.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

tests/integration/api_service_test.py

@@ -376,7 +376,7 @@ def test_create_service_with_network_attachment_config(self):
             'dockerpytest_1', driver='overlay', ipam={'Driver': 'default'}
         )
         self.tmp_networks.append(network['Id'])
-        container_spec = docker.types.ContainerSpec(BUSYBOX, ['true'])
+        container_spec = docker.types.ContainerSpec(TEST_IMG, ['true'])
         network_config = docker.types.NetworkAttachmentConfig(
             target='dockerpytest_1',
             aliases=['dockerpytest_1_alias'],