Fix the problem with impure tests changing backend_fixtures

One of the tests is impure, changing backend_fixtures, which breaks test_model.
This commit makes backend_fixtures to return a deep copy of backend_fixtures_json isntead of returning them directly.

python/tests/fixtures/backend_fixtures.py

@@ -14,6 +14,7 @@
 #   limitations under the License.
 #
 
+import copy
 import json
 import os
 
@@ -80,4 +81,4 @@
 
 @pytest.fixture
 def backend_fixtures():
-    return backend_fixtures_json
+    return copy.deepcopy(backend_fixtures_json)