MNMG Leiden - Define C++ unit tests

[ChuckHastings]

Define C++ unit tests for Leiden. Mimicking the basic tests for our algorithms might be sufficient. The tests should contain some tests where the clustering differs from Louvain, especially for the different examples where Leiden is trying to account for deficiencies in the Louvain clustering.

Tests should include both SG and MG tests.

[github-actions]

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[ChuckHastings]

Completed as part of the Leiden implementation