iinfo() tests should ignore undefined unsigned dtypes

[asmeurer]

iinfo tests fail with pytorch, which is missing these dtypes:

FAILED array_api_tests/test_data_type_functions.py::test_iinfo[uint16] - TypeError: iinfo(): argument 'type' (position 1) must be torch.dtype, not _UndefinedStub
FAILED array_api_tests/test_data_type_functions.py::test_iinfo[uint32] - TypeError: iinfo(): argument 'type' (position 1) must be torch.dtype, not _UndefinedStub
FAILED array_api_tests/test_data_type_functions.py::test_iinfo[uint64] - TypeError: iinfo(): argument 'type' (position 1) must be torch.dtype, not _UndefinedStub