drivers: base: Add basic devm tests for root devices

The root devices show some odd behaviours compared to regular "bus" devices
that have been probed through the usual mechanism, so let's create kunit
tests to exercise those paths and odd cases.

It's not clear whether root devices are even allowed to use device
managed resources, but the fact that it works in some cases but not
others like shown in that test suite shouldn't happen either way: we
want to make it consistent and documented.

These tests will (after the following patches) ensure that consistency
and effectively document that it's allowed.

If it ever turns out to be a bad idea, we can always roll back and
modify the tests then.

Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/r/20230720-kunit-devm-inconsistencies-test-v3-1-6aa7e074f373@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>