iio: test: Add test for IIO_VAL_INT_64.

Add test for newly introduced type IIO_VAL_INT_64.

Signed-off-by: Andriy Tryshnivskyy <andriy.tryshnivskyy@opensynergy.com>
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20211105100501.1904-3-andriy.tryshnivskyy@opensynergy.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>