[hooks] build EnsureSymmetricRelations{Add,Delete} dynamically Instead of setting the __select__ attribute at registration time, which confuses the filterable_selectors cache when running different tests (with different schemas) in the same process, just make entirely new classes. Closes #8785708.

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# uri,name,knows

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http://www.example.org/alice,Alice,

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http://www.example.org/bob,Bob,http://www.example.org/alice