test_can_cast should not disallow cross-kind casting

[asmeurer]

test_can_cast currently fails with NumPy because NumPy allows cross-kind casting. However, the spec only says that mixed-kind casting is undefined (https://data-apis.org/array-api/latest/API_specification/type_promotion.html), and can_cast just says it should follow the type promotion rules (https://data-apis.org/array-api/latest/API_specification/generated/signatures.data_type_functions.can_cast.html). So it should not test this case. If anything, it should check that if can_cast does return True for a mixed-kind cast, that such a cast is actually allowed on an array of that dtype.