Fix factorial(0)

Bug: 24575798

Make factorial(0) return 1, as intended.
Add a few factorial tests.
Fix targetPackage spec so test is actually runnable again.

Change-Id: Ibb8d827f0325999c6b5ed49a939e1532ddf55639
(cherry picked from commit 6951591806f46405564a6cb868a88d0daf6764d2)
4 files changed