iio: test: add basic tests for the iio-rescale driver

The iio-rescale driver supports various combinations of scale types and
offsets. These can often result in large integer multiplications. Make
sure these calculations are done right by adding a set of kunit test
cases that build on top of iio-test-format.

To run these tests, add the following to .kunitconfig
	$ cat .kunitconfig
	CONFIG_IIO=y
	CONFIG_IIO_RESCALE_KUNIT_TEST=y
	CONFIG_KUNIT=y

Then run:
	$ ./tools/testing/kunit/kunit.py run --kunitconfig .kunitconfig

Signed-off-by: Liam Beguin <liambeguin@gmail.com>
Reviewed-by: Peter Rosin <peda@axentia.se>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220213025739.2561834-7-liambeguin@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>