GH-4785: unit test for timeout / deadlock in FedX Join

[aschwarte10]

This commit provides a unit test that reproduces a deadlock scenario. The issue is somewhere caused in the join with NUnion, and specifically the union parts having relevant statements in multiple sources.

Note that the dataset is really small (42 triples, 40 relevant ones contributing to the result)

For some details and explanation please refer to my comment in the issue at #4785 (comment)

GitHub issue resolved: #4785

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