Conformance suite: rewrite several tests regarding type-parameter defaults to not assume that type checkers will infer a class object as having a type[] type

[AlexWaygood]

Many tests in conformance/tests/generics_defaults.py and conformance/tests/generics_defaults_referential.py currently assume that a type checker will infer a class object as having a type[] type. But type[int] doesn't mean "the class object int", it means "the class object int, or any subclass thereof", which is insufficiently precise in many cases for ty's purposes. ty therefore infers "class-literal types" for class objects rather than a type[] type.

If a type assertion fails due to the actual inferred type being a more precise, unspellable subtype of the asserted type, ty will emit assert-type-unspellable-subtype instead of type-assertion-failure (ty's inferred type of <class 'int'> is an unspellable subtype of type[int]). We may switch this error code off when we add ty's conformance results soon. But several of these tests are assertions involving non-fully-static types, so it is not true that the actual type is even a subtype of the asserted type in these cases (non-fully-static types do not participate in subtyping).

This PR reworks these tests to make the assertions more ty-friendly in these respects, while keeping the assertions just as precise as they were before.