of/unittest: Remove test devices after adding them

The of_platform_populate() test cases don't remove the test devices
after they are added. Fix this by adding tests for
of_platform_depopulate().

At the same time rework the selftest() macro to return the test result
value. This makes it easy to use the macro inside an if() condition.

Signed-off-by: Grant Likely <grant.likely@linaro.org>