[TEST] Add integration test for `test_extract_nodes_edges`

[joshua-goldstein]

In https://github.com/dgraph-io/pydgraph/pull/200 we added a new function test_extract_nodes_edges that has a unit test, but not an integration test. It would be nice to have an integration test because if the dgraph response format changes, the test would fail, and we would know to change our unit test as well.

[github-actions[bot]]

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