cmake: Get unit tests passing CI again

The following commit:

commit 407b49b3 (refs/bisect/bad)
Author: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>
Date:   Wed Feb 12 15:00:46 2020 +0100

    cmake: use find_package to locate Zephyr

breaks as we don't find the ZephyrUnittest package.  For now revert to
the old means until a proper fix can be made.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>