iio: test: format: follow CONFIG_ naming convention

The KUnit documentation indicates that all KUnit Kconfig entries must
match CONFIG_<name>_KUNIT_TEST: where <name> is the name of the test
suite. Rename the IIO_TEST_FORMAT configuration accordingly.

Signed-off-by: Liam Beguin <liambeguin@gmail.com>
Link: https://lore.kernel.org/r/20220710013109.3349104-4-liambeguin@gmail.com


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