iio: test: rework Kconfig to support modules

Rework the IIO test Kconfig to support building KUnit tests as modules.
This lets users execute tests at runtime in addition to the usual
tools/testing/kunit/kunit.py script.

Signed-off-by: Liam Beguin <liambeguin@gmail.com>
Link: https://lore.kernel.org/r/20220710013109.3349104-6-liambeguin@gmail.com


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