cmake: moved unittest.cmake to Zephyr CMake modules dir

Move the unittest.cmake to the new Zephyr CMake modules dir.

This allows us to have a single Zephyr CMake package and load unittest
module as: 'find_package(Zephyr COMPONENTS unittest)'

This unifies the way Zephyr package is sourced and removes the need for
a dedicated ZephyrUnittest package.

Deprecate the use of: 'find_package(ZephyrUnittest)'

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