iio: test: Add KUnit tests for iio_multiply_value()

Add KUnit tests for iio_multiply_value().

Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Hans de Goede <hansg@kernel.org>
Link: https://patch.msgid.link/20250831104825.15097-6-hansg@kernel.org


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