[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.
# uri,name,knows
http://www.example.org/alice,Alice,
http://www.example.org/bob,Bob,http://www.example.org/alice