cmake: make find_package(ZephyrUnittest...) REQUIRED

This commit is a followup to PR #25808 which updates the tests to
ensure the REQUIRED keyword is also used for the ZephyrUnitest package.

This provides a better error message when building with CMake and
forgetting ZEPHYR_BASE or not registering Zephyr in the CMake package
registry.

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