tests/interface: Add a (currently crashing) BMenu "concurrency" test.

Adds/removes items as rapidly as possible from the menu from the
main thread while the menu is open. That part works. Then it deletes
all of the BMenuItems and then closes the menu, which crashes,
as the BMenuItems do not notify the BMenu they are being deleted.

I tested this on BeOS and it seems that this model crashes there also
(looking through the code comments, it seems there are a number of
testcases found throughout the years like this.) Removing the items
before deleting them indeed fixes the crashes on both BeOS
and Haiku.

Change-Id: I8624f966bdc17030ddca05b97aa57b518ab420c5
Reviewed-on: https://review.haiku-os.org/540
Reviewed-by: waddlesplash <waddlesplash@gmail.com>