[test] Fix docker network object handling

The network created during a test run was not properly deleted.

beat/core/test/__init__.py

@@ -57,6 +57,7 @@ DOCKER_NETWORK_TEST_ENABLED = (
     os.environ.get("DOCKER_NETWORK_TEST_ENABLED", False) == "True"
 )
 network_name = os.environ.get("DOCKER_TEST_NETWORK", "beat_core_test_network")
+global network
 network = None
 
 # Setup the logging system
@@ -90,6 +91,8 @@ def setup_package():
     if DOCKER_NETWORK_TEST_ENABLED:
         import docker
 
+        global network
+
         client = docker.from_env()
         try:
             network = client.networks.get(network_name)
@@ -112,5 +115,6 @@ def teardown_package():
     shutil.rmtree(prefix_folder)
 
     if DOCKER_NETWORK_TEST_ENABLED:
+        global network
         if network:
             network.remove()