Fix broken test due to BUSYBOX -> TEST_IMG rename
Fix broken test due to BUSYBOX -> TEST_IMG rename

The BUSYBOX variable was renamed to TEST_IMG in
54b48a9b7ab59b4dcf49acf49ddf52035ba3ea08, however
0ddf428b6ce7accdac3506b45047df2cb72941ec 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after that change, but was out of date, and therefore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 <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

vdemeester/docker-py,vdemeester/docker-py,docker/docker-py,docker/docker-py