tests: source Zephyr before project() call

Fixes: #73831

find_package(Zephyr) should be called before first project() call.

Zephyr package will test and force-set the correct toolchain, especially
the C compiler.
The project() will also set the C compiler, if not set already.

If project() is called first, then conflict arises on the C compiler
selection and thus the following message is seen:
> You have changed variables that require your cache to be deleted.
> Configure will be re-run and you may have to reset some variables.
> The following variables have changed:
> CMAKE_C_COMPILER= /usr/bin/gcc

This cache deletion results in other errors, such as a missing BOARD
setting.

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