cmake: remove CMake policy CMP0000 and CMP0002 in unittest.cmake

Two CMake policies has been removed:
CMP0000 was introduced in CMake 2.6 and set to OLD, but with all Zephyr
        CMake files using cmake_minimum_required() then there is no
	reason for having this policy to OLD.
CMP0002 was introduced in CMake 2.6 and was set to NEW.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>