Improve stubtest handling of expected dunder methods (#21764)

Stubtest currently suppresses diagnostics regarding missing dunder
methods whenever the runtime attribute is a `WrapperDescriptorType`.
This is because `WrapperDescriptorType` dunders are often not actually
callable at runtime, so suppressing these diagnostics avoids many false
positives. However, it also hides genuine omissions such as
`collections.deque.__rmul__`
([typeshed#16055](https://github.com/python/typeshed/pull/16055)).

This PR keeps the existing suppression in the general case while
reporting missing methods that the stub gives us a reason to expect:

- If a stub class has a `__add__` method, a diagnostic regarding a
missing `__radd__` method is probably _not_ a false positive
- If a a stub class has a `__lt__` method, a diagnostic regarding a
missing `__gt__` method is probably _not_ a false positive
- If a stub class inherits from `Mapping`, diagnostics regarding missing
`__or__`, `__ror__` or `__ior__` are probably _not_ false positives
- If a stub class inherits from `Sequence`, diagnostics regarding
missing `__mul__`, `__rmul__` or `__imul__` methods are probably _not_
false positives.

There may be more heuristics we could add similar to these. These were
just the ones I/Codex could think of.