BOutlineListView: fix ItemUnderAt and accept NULL as superitem

ItemUnderAt was returning items that were not under the superitem (this
was fixed for EachItemUnder in hrev52210).
Make a NULL superitem mean the parent of the topmost items. Despite not
being explicitly documented in the BeBook, that's how BeOS works, the
MenuWorld test app uses it and, well, it's handy.

Change-Id: I2551e8ce874a6238c5e5fb1eb742e68e62d3928a
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7359
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>

Revive unit tests

* Fix build on 64-bit.
* Test suite can now run from start to finish.
* Changes in OutlineListView test need an explanation:
for some reason when BApplication is created on heap, be_app is not
reset. This causes other tests to crash due to second BApplication
object being created.

Change-Id: Ic7d55f4622192f83572bfd83fa37f1f5fd641e22
Reviewed-on: https://review.haiku-os.org/465
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

Add unit test for BOutlineListView.

Change-Id: Icad7e7c659141fae1bb6e52571f001c9f561a446
Reviewed-on: https://review.haiku-os.org/451
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>