[red-knot] 'is_equivalent_to' is an equivalence relation (#15488)
## Summary
Adds two additional tests for `is_equivalent_to` so that we cover all
properties of an [equivalence relation].
## Test Plan
```
while cargo test --release -p red_knot_python_semantic -- --ignored types::property_tests::stable; do :; done
```
[equivalence relation]:
https://en.wikipedia.org/wiki/Equivalence_relation
