cmake: Relocating Zephyr Unittest CMake package.

Fixes: #23872

Relocating Zephyr Unittest CMake package to ensure that
HINTS ${ZEPHYR_BASE} in
find_package(ZephyrUnittest HINTS ${ZEPHYR_BASE}) works correctly when
the package has not been exported to CMake user package registry.

This ensure that the new package functionality is fully backwards
compatible on systems where the package is not exported and ZEPHYR_BASE
is set.

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